I can understand your point exactly in your opening statement, and I agree that Abyssea and the level cap made a lot of events dead for large groups. But it was just that, large groups. The state the game was in before the level cap raise, at least from my experience, not so many people were doing end game events because they had what they wanted. Dynamis numbers were dropping lowere and lower, same for Limbus. And a few times we were pushed for people to enter Salvage with.
With the level cap raise that all changed, it's very rare now that you need a full aliance to do the older events. I've done Nyzul in a trio, duod Limbus with a friend, and loads of other stuff that I probably couldn't have done before. If you want to do all the events which were older, just get a few like minded people and do them. It's not as hard as you might think if you ask people who are currently not in the same Abyssea run as you or old friends.
As far as making some of the older armor such as Artifact or Relic upgradable, most pieces in this game are situational and there are situations where pieces from those sets are still the best. As a Summoner I use the artifact legs for physical blood pacts since there isn't really much better. I think even the Empyrean+2 legs wouldn't give more accuracy than those. Armor has always been situational in this game and as of yet some of those pieces are the best for that situation, and if people Abyssea their job and don't go get them, sucks for them.
As for Abyssea ruining how people play the jobs, I didn't see that mentioned at all in the first post, and it's all about the player not the method used to level. I've had friends who Abyssead Monk or Ninja, and they were amazing at them. I've had friends who leveled the same jobs out of Abyssea before the cap and weren't so good.
tldr; the events can be low manned, don't need linkshells/large groups for them