I am sure that many areas require parties, good items require trading and basically luck, etc etc, no? I used to play a lot of online games with Lineage 2+MU Online, but that was a while ago and I certainly got my fill (even though I am sure it would be hella enjoyable if I got into it now, just a lot too time consuming).
There is also the fact that the game will be constantly updated, so you never really "beat" it.
Unfortunately, yes. Although it isn't noticeable in the first, let's say, 10 to 15 levels, there's a point where basically the game keeps halting your "singleplayer" progress to push the MMO aspect onto you; mainly dungeons and raids, which the main story requires you to do, but you do require other players in order to tackle them. Depending on the dungeon you can sometimes take up to an hour not doing anything at all just waiting for a party to gather and do it. It's even worse when you're paired with people that has done that specific raid numerous times, as they skip all the cutscenes (it doesn't affect you, you basically prevent them from going on if you don't skip the cutscene yourself) or bomb the chat, or just try and go ahead and get to the boss ignoring the rest of the dungeon. This even applies to some specific bosses, whose raid sequence is just the boss fight itself (story-wise, you have to fight certain espers, like Ifrit), but yet the game expects you to wait for a party to gather, and that takes a lot of time.
It doesn't really let you get to your own pace beyond that point, and you have to just play as an MMO whether you want or not. Best case scenario would be joining a guild and make friends that would let you, but then again that's a MMO staple too.
Beyond that, the main story (or certain sidequests) try their best to keep the singleplayer narrative going, but sometimes they make some incredibly boring sequences just to push yet another MMO aspect onto you, like when the game forcibly makes you choose a Grand Company between the main cities to join, but also forces you to visit each one of the big cities to watch a speech about higher causes that tries to convince you which Grand Company to join.