Nobody measures the worth of a game by the amount of worthless content in it. The main story is basically your usual MMORPG fare that's comparable to fanfiction, except that'd be rude to good fanfiction authors to compare this (and most MMORPG) to it. Therefore the ability to waste a few hundred hours going through the main story, possibly multiple times, is not considered worthwhile. Note that if a single player RPG had a story as bad as FF14 most people would never buy it in the first place.

Just because you wasted a lot of time playing a bad game, doesn't mean the game was actually worth it. It's true in MMORPG most players have expectation to have to waste some time to get to the good parts, but that's really just a fallacy. It's not like having the main story of FF14 being so bad somehow improves the other aspects of the game.