I think that endgame should have heavier requirements on cross-class abilities - thus requiring players to level multiple jobs.
Most of the 'hard-core hardcores' have a single job at 50 and the support classes at their min req level for that job.
On the larger issues of the game:
I think it was a big mistake to make the journey from 1-50 a practical faceroll.
When you look at it, the majority of content/dungeons/quests/crafted items/etc that was created for this game is for levels 1-49, and it is all rendered irrelevant in the blink of an eye as players rush past it to endgame.
I don't think an FFXI-style grind is an option either, but they should have split the difference so that people would have to spend time progressing and enjoy the wealth of <50 content.
This approach would buy the devs ample time for expansions as only legacy players/ultra hardcores would be able to plow through it all, while mid-cores and casuals would still have lots to do. From a developer standpoint, you have to accept that some people will get bored, it's unavoidable. But Legacy players will stick with the game long-term no matter what, they've already proven it~ Conversely, most hardcore MMO players are downright fickle, no matter what you do. Thus they tend to be unreliable as a steady source of income for MMO games. Keeping your generic hardcore MMO player as a subscribers depends largely on whatever the current MMO trends are and where they think they can show off their e-peen the most effectively.
The real danger emerges when mid-cores and casuals become bored. This happens when content is either insufficient (not a problem for ARR) or extremely difficult/frustrating (again, not a problem for this game), or too quickly/easily completed (see ARR leveling 1-50).
This is a difficult situation to rectify as players are highly resistant to any changes to content other than nerfs, and some will even complain about that...