First off Abyssea is an issue, not level sync, that is a design flaw, and not a flaw with a completely irrelevant feature in Level sync. Its also imbalanced. FFXIV is not anywhere NEAR as imbalanced as FFXI's system, so it won't take that long to change mob balancing (There is not one single 'mob' that completely overwhelms the population, and if there was it would not be a problem with level sync, those mobs would be exploited no matter the level if they exist over the course of the games levels), at that points its mob distribution, there are now tons of camps with lots of mobs, and those mobs all spawn very fast, so there won't spawn balancing issues that FFXI has (Dungeon mobs spawn in 15 minutes, 'some' field mobs spawn in 12 minutes, the rest spawn in 5 minutes unless your in abyssea, in which case all spawn time fail in comparison). The only thing they have to work on combat stat balancing and gear balancing around the system and they have said they were going to look into implementing level sync way before the first part of the combat overhaul was put into place, there is no sudden 'change' that needs to happen in the design process like its going to add that much to dev time.
Secondly you just referenced the mmo king of community disenfranchisement with World of Warcraft. You either had friends or you leveled solo, pick groups were very rare, this got even worse as time went on. It was solo to the cap and hope you find a good guild. What did they do to remedy this? They implemented a system that 'synced' you into a group for a specific instance around your level range, the trade off is you didn't need to go out into the world anymore, you hardly ever played with people in your own server, and when you hit the highest level you did the same thing over and over again and you either did it with friends or did it with other players either on your server or not on your server, but you didn't need to engage in it. You want to tell me the future isn't level sync, complain about how it destroys communities and you bring up WoW as your counter? And as for SWTOR and GW2. They both have a form of syncing. PvP is completely level synced unless your in the open world, and if you are not synced, you need to group up with people who happened to playing in your area, otherwise you do everything solo, EVERYTHING. Its unknown how the community will flourish in the end game, but the leveling experience leading up to it is far from a great future in community. Tera I haven't played nor heard enough about to make an opinion but GW2 they have storyline sync where people of a variety of levels can participate in the same content and get something out of it.
The point is level sync isn't the problem with community, there are far more damaging features that can do that, the key is implementation and FFXIV is far more ready for level sync then FFXI was when it was introduced, and for FFXI it has only gotten worse, but that is not because of level sync, its because they implemented another EXP system that completely broke all the exp mechanics up to that point in abyssea, and then later Heros of Abyssea quest system.