Lets go to the other extreme. Lets say gear lasts 7-8 years before something superior comes. How many would still be playing today? At some point, horizontal progression becomes silly. As a FFXI player from 2003-2010 and endured all the hardships and play styles, I cannot go back to this type of thing. I need a new goal every few months, I like to explore new dungeons, take on new bosses, and explore new areas at a reasonable pace. FFXI did a fine job abusing the RNG/low drop rate system to keep you going, but it doesn't work as well in modern times. Look at 1.xx, CC/AV were the only dungeons to play for 9-10 months. People got sick of it within 3-4 months, people hated progress being haltered by extremely low RNG. It caused people to quit. You can't expect people today to play only certain content for a long period of time. Making long period content creates these frustrating RNG systems that do cause people to quit.
Things like vertical progression are popular over horizontal is because people naturally like feeling like they are making progress in something every day. Creating horizontal means you have to drastically hold back the rate on how people progress, which in the long run, makes it feel less appealing to people. This was a major complaint with friends of mine who played other MMO's during FFXI's prime and could not get into it. Extreme horizontal (which FFXI was) is a very unique taste that only few realistically enjoy. Making content intentionally last the long run pretty much shows that the developers don't have many ideas going on and dragging on content seems like lazy development. FFXI gets a pass on this because of PS2 limitations, it can't bring out content at the pace ARR does. So it recycles content and adding new rules in certain content, or reusing areas to make new content (Abyssea, WoTG). Gear rarely gets put in because adding pieces constantly would take up a lot of space on what little that PS2 HDD has. So the majority of new gear that does come out is a recolor or slightly altered design of current gear. Its not to say it is 100% the reason, but development and concepts of FFXI were built around knowing they had to work with limited space and knowing they could only go a certain extent on adding new content.
It was fun for what it was, but player mentality does not favor this play style anymore and the only people I see complain about a lack of horizontal progression is FFXI players. You can point a finger at me and call me out, but I am pretty sure SE did some thorough studying of what play styles people prefer, including their current fan base at the time before making decisions that led to the game we play today.
During my play of FFXI, going through sky/sea every day, hunting down NM's to get the items to unlock the bigger bosses, or any content like Dynamis. I never truly felt achievement in gaining an item, more like relief. For the amount of time and effort I put in, I always felt obligated to get what I am looking for, not held back by other players necessarily, but the rules of the game were pretty punishing. I never felt like my effort and reward were ever properly balanced. I could go weeks without making any form of gear progress on my jobs. Some may be able to endure this, and I did, but the average player today will not settle for that. When people argue that "They will get use to it", they won't. Even FFXI players (least the ones I played with) were only truly tied to the FF name itself, and found certain requirements and time sinks beyond ridiculous.
I say all these negative things, but did I hate FFXI? Of course not, for all the things me and my friends dealt with over the years, we did enjoy aspects. Such as a colorful world, an interesting job system, combat (for it's time) was fun, really well written story, and so on. Stuff like Assault and Nyzul Isle were really fun, their only issue was the balance of reward and effort. Travel a 100-foot tower just for nothing to drop, content that was relatively difficult and took a lot of effort to accomplish, just for nothing to drop. A sad tale indeed for anyone.
It isn't fair to say nothing from FFXI wouldn't work in FFXIV. I personally would like to see content like Nyzul Isle and utilize the guildleve system to expand upon open world content. I'd like to see jobs like Samurai and Corsair make it's way to FFXIV with it's own take on it. I'd like to see Chocobo Racing return. They are ideas I like and I believe can work in the context of FFXIV's built and established system.
I do find SE's current system is in the right direction, despite some small flaws. They are making vertical progression with a small mix of horizontal, by offering various content that drops high tier. Rather it be raids, primals, crafting, or mythology farming, there is various ways to make progress during the day. This to me really feels like a "having your cake, and eating is also" scenario. Of course some in here will not agree with me.
You are 100% obligated to your opinion if you favor FFXI's style over ARR's. However, all argument in here is completely theoretical and realistically the developers would not completely change and alter the concepts and mechanics of the game. They already have development in content all the way to the expansion, which is also in development also right now. I also hear the argument "FFXI isn't what is use to be, I wanted FFXIV to be the answer to it." Well, reality check, it isn't, and likely not going to be if you are that dissatisfied with the system. Like I said before, the only FFXI experience you are going to get is in FFXI itself. I already see the FFXI developers trying to bring some old content back. You would have a better chance of getting your style of FFXI back by going to their forum and expressing your dissatisfaction for the direction it took.
ARR is it's own thing, it had no obligations to FFXI nor 1.xx players in making a game suited for them. All they owed to us was making a quality game that befitted not only the Final Fantasy name, but the MMORPG genre as a whole, which I 100% believe they accomplished rather if you agree with me or not.