You sort of answered your questions yourself. It's "reason". No one of sound mind would "buy their way to the top, then abandon the game for lack of content".
Let's assume someone isn't the brightest and does act this way: That is a player you don't want to stay anyway. He leaves on it's own. How would he have friends ingame? As you stated, he bought his "way to the top". No player interaction needed.
(Sarcasm: even though i still don't know which vendor is offering DL-gear and relics.)
Now here's something that really amuses me: XIV is exactly as you don't want it to be.
It's a standard MMO by all means! If you disagree, what makes this game differ from other avatar-based mmo's regarding game mechanics?