I wouldn't say that the setting of Monsters Inc is uninteresting. The factory is far from banal and Monstropolis itself is a very lively, colorful, fun environment that was created with a pretty deep level of detail provided we don't spend a huge chunk of the movie exploring it.
And then when we shift into the factory, we're introduced to an incredibly creative environment that yes resembles something you'd find in the human world, but on a completely different scale, full of different sight gags and twists. It's extremely well suited to exactly what they're doing with it which is giving it a land based on the city, and then creating this coaster that places you within one of the most exciting and fascinating environments of the entire film.
Yes, a dark ride can work if done well. It's yet to be done well as a dark ride, though. Tokyo's is decent but far from great, and California's is very substandard. But the property is far more flexible than just a dark ride, and we shouldn't be hamstringing it to only that style of attraction. It can work in different forms, and a coaster is most certainly one.
I've watched this film well over a couple hundred times in my lifetime (there was a stretch of time when I was a little kid where I no joke for around a year watched it at least once a day). I understand this movie deep, it's my favorite film of all time, and I would NEVER advocate for something I didn't think would work with it.