It's fine solo. I've been playing it solo for about 40 hours. Some people have been playing it solo for hundreds of hours.
I don't know why the publishers have chosen to pretend it's multiplayer only. It isn't. You can play either way. It works equally well as a SP or MP game. You can even have a SP world and a MP world with the same character and swap between the worlds at will, taking whatever is in your inventory with you.
This has put me off up to now. It’s been something I like the look of, but honestly just thought it was something I would miss out on, as I game solo now. I much prefer to do things at my pace.