Let me refute each of those.
It's been around for over a decade with consistent development and support from its parent company. This is why it makes so much money: not because it is a good game, but because it is a long standing MMO that gets continual development. MMOs fail when they stagnate. You have to keep developing a game in order to keep people playing it. This is the *one* thing that FFXI has done well, but it has nothing to do with the game design and more to do with the business management.
The fact that it is a subscription game falls within this same category: people are willing to pay $15 a month if the game gets updated regularly, even if the content is mostly crap. Novelty is a huge factor.
On top of this, it also arrived when the MMO market was still pretty immature (it was released in 2002; WoW, which during the Vanilla days, made a *crapton* of horrible design decisions because they didn't know any better, was released in late 2004) and, much like WoW, got a *lot* of attention because of the cache of its name and the accompanying series. A preexisting fan base means that the developers have immediate access to a massive potential player base without having to put forth the effort to get attention. It's why WoW, SWTOR, EQ2, and WHO all had very large release numbers. FFXI had more of this than any other MMO ever released because the FF series has had a *very* loyal playerbase going back for over *20 years*.
As to the people saying that ARR should be more like XI, that's not due to the quality of XI; it's due to the fact that the average person is deeply uncomfortable with change. ARR has a dramatically different system than XI *or* XIV1.0. Even if the system were absolutely perfect in every possible way, people would get upset because it deviates from what they were used to in previous games. It doesn't help that a lot of people live under the absurd notion that you never have to make compromises when developing certain systems. It's also why FFXI has so many players; there are 2 basic varieties of players: long haul gamers, which will play for years as long as you don't screw the game up, and short term gamers, which will play for a couple months before they get bored. Once a game hits the 3-4 month mark, the short term gamers leave and the long haul gamers stick around and, most of the time, they'll stick around *forever*. The combination of these two elements of the same basic idea converge to explain XI's "superiority" to XIV: you've had a bunch of people that have played a given game for years on end with a poorly constructed system that they are incredibly used to switching to a new game that uses a different system. It's not that XI is superior to XIV; it's that those people are more used to it and they want anything they play to be either a minor deviation from what they're used to or simply the same system with a new story or development push.
One of the threads you're referring to talks about the lack of elemental weaknesses to exploit, which has been something that's deeply set within the FF series. The problem with this (which WoW found out very early on in Vanilla, with all of their endgame raids devoted to basically a single element, the major ones of which were fire) is that it prevents the creation a relatively simple but still compelling play experience. In effect, you're either going to two of these three: a manageable number of abilities, a system that allows for appreciable elemental weakness/strengths, and an interesting and compelling implementation of the combat system. Look at how BLM is designed: it's one of the most fun implementations of an offensive caster that I've seen. It's elegant in its design and fun in execution. The only way you'd be able to use that same playstyle while allowing for elemental weaknesses and strengths would be to create 3-6 times the number of abilities, since you'd need to allow access to all elements while preventing one of the phase aspects of BLM to be rendered redundant (i.e. you would need to allow for variant element Astral Fire spells, variant element Umbral Ice spells, and variant element Thunder/Thundercloud spells in sufficient numbers to cover all elements). Even so, you would have incredible amounts of redundancy when you don't have to worry about elemental resistances weaknesses and, when you do, since there are so many spells, you have to tweak your bars/rotation/keybinds to cover that specific set of elemental weaknesses and strengths. It preserves the playstyle and implements the weakness/resistances, but you end up with so many spells and so much non-combat ability management that it becomes untenable for anyone who doesn't enjoy regular UI manipulation.
There are a *lot* of people that misunderstand certain aspects of ARR and *why* they are good design decisions implemented because the developers were aware of the bad design decisions of other games. ARR has single role jobs/classes because it gives them stronger control over role population. ARR has very little intraclass/interclass customization because other games that *do* allow for such customization just end up with optimized cookie cutter builds that everyone goes with and anyone that deviates from them is denigrated for being substandard; in effect, the only choice you have in those games is whether you're going to be effective and identical or substandard and different.
ARR did away with the aspects of MMOs that developers and players wrongfully considered to be good design. It has been found out that talent trees are not good game design. It has been found out that allowing for multiple roles within a single class that, while not outright *bad*, is not especially *good* game design. ARR doesn't have open world end game or open world PvP because, once again, it's not good game design. So very, very many things about ARR were done because the devs took an objective look at what benefits and what baggage those elements of play brought with them. Rather than bringing them in because they sounded good or because that's just how things had always been done, they actually thought about the problems/goals and considered variant/non-standard designs that would allow them to do so with fewer problems while fulfilling the same goals (especially fulfilling those goals without the pretense and self-delusion of illusory choice proffered by so very many "customization" schemes).
Hell, I honestly believe that the implementation of ARR's endgame and group content development is the absolute best of any MMO I've ever played: the primal trials are boss fights without the arbitrary trash cluttering everything and you can repeat them as much as you want (which is *awesome* and largely without precedent); coil is a standard weekly dungeon with all of the stuff you'd expect implemented in a way that allows comparatively casual players to take part as well (since it doesn't lock you to a group); CT is a very interesting variation on the standard weekly dungeon because you can run it indefinitely but you can only get a single piece of loot per week. Anyone that claims that ARR's endgame should be more like FFXI needs to be put in a sack, have the sack thrown into a river, and have the river lit on fire. Camping certain spawn points for hours on end and getting into boss fights intended to last hours at a time for the "fast" kill is not good design. Sitting around doing nothing isn't fun, especially when your time can be quickly wasted because someone else got the first hit instead of you.
Basically, there are 2 reasons that FFXI is still considered a success: it's part of the FF series and it's been around long enough that there is tremendous inertia in the playerbase. Neither of those have anything to do with it begin a good game. *Many* commercial successes are completely bland games that were just marketed well or require negligible development with a ludicrously loyal and/or large potential player base (just look at people that play CoD or any sports video game: sports games are still basically the same as they were in the NES days and FPSs haven't really changed since Doom; the only real difference is just the improved graphics).