While I agree with some parts, I don't agree with others. Secondary class to 15 was fine for flavor, but the Cross-Class system was atrocious. Not only was it never more than a worse imitation of the SubJob system from FF11/Tactics, the things you got from other Jobs were generally in one of two categories - necessary or worthless. There was almost NO in-between. It wasn't "Level THM on WHM if you want to get this nice thing, but you don't need it". It was "If you don't level THM on WHM, you won't have Swiftcast, which is mandatory. Better get to leveling this thing you now hate with a passion."
...I STILL hate THM/BLM. To this day! I disliked it then, and making me level it did NOT make me like it, it made me HATE it, and I STILL hate it because of that. What, 8 years later, and I STILL get mildly annoyed just thinking about it. (Arcanist was different since you could use Topaz/Titan to tank for you, which could be fun, but THM was just...god that class is SO AWFUL to level. Even now I hate BLM. Worst designed Job in the game; people that say otherwise are on crack or something, I swear... <_< )
And when they figured out it was a bad system and rightly replaced it, then the level 15 thing didn't make sense to keep around anymore. The lore was never developed (why does an Arcanist need to know basic Conjury in order to learn Scholar arts?), so the only reason for the system was to tie into the Cross-Class action borrowing, which no longer existed.
Classes themselves weren't even good design, they were incurred debt from 1.0, which originally did not have Jobs. If you didn't know that, you read that right; 1.0 did. Not. Have. Jobs. They were added relatively late in the "expansion's" history, which also completely changed the classes. Do some googling into it and you'll find CNJ was almost more a THM with healing spells, and THM was a weird hybrid of umbral elemental magic and some weird utility stuff and SORTA blood mage healing weirdness (that...I don't think worked well, hence the reason it was removed). There was also a class that had a shield as its weapon. And no weapon. It was entirely for learning stuff to cross-class to GLD (and THM/CNJ if they wanted to use shield bash...because that was a thing).
It's why they've never added another class after Rogue with Ninja, because it was a pretty bad design they were kind of saddled with. While I do agree that in theory they could have done something else with it, it wasn't designed to do so, and didn't work well.
And the ultimate problem with Cross-Class skills, and later the first version of Role Actions (where you had a selection and could only pick 5 at a time), is the same issue WoW has had with Talent trees for ages, leading to MULTIPLE entire reworks of the talent system over time; there's almost always "right" and "wrong" choices and cookie-cutter builds, leading to an illusion of choice where many choices are just newbie traps.
...which isn't a bad thing if you're in an old school MMO where no one knows what they're doing and no one cares in the year of our lord 2004, but a big problem in the year 2023 with parsing and DPS checks and theorycrafting and people calling other people griefers for not having the right ones selected. Or, to quote an oft presented phrase, we as gamers have "optimized the fun out of everything". So things that are lore and flavor are seldom looked at fondly if they result in a DPS loss in the sterile, overly-mechanical view of the world we now find ourselves in...sadly, because I think it's a crap way to view games and the world, yet here we are.
.
I do feel like they could and should do more on the lore and RP side, push back some of the homogenization, etc. But not everything they got rid of WAS good. Many of the things they got rid of were bad, that's why they got rid of them.