Even FFXIV (my current drug of choice) has limits to what you can do as a solo player. Even the MSQ - the most single-player content in the game - requires you to group up with others every other level and do dungeons. It’s a mostly seamless process with all the duty finder enhancements and roulettes that bring existing players back to the old content, but it’s still there.

The benefit of a clan/guild/free company can’t be understated. They will provide entertainment, support, and access to a set of content that’s otherwise very hard to do.

From ShB onwards, you can now use the Trust system to solo dungeons and MSQ events. I'd be willing to bet they will port this to older dungeons once New Game+ comes out, although I am not sure if they'll ever do this with roulettes.

Trusts do alleviate a portion of it, but there’s entire swaths of content you can’t do solo (including all of the MSQ-blocking Trials).

I like trusts, but they’re definitely a band-aid for a very specific issue (DPS queue lengths).