.... Yeah, that's pretty much true, and for very good reason. Every time an interface changes, it forces people to stop using the system and figure out the new interface. As such, while it is possible to make changes that are beneficial enough to justify the cost, UI changes are very expensive and you'd better have a very good reason to justify breaking everyone's workflow. And by that standard, yes; XFCE is nearly perfect. It's been on the 4.x series since 2003, and going by screenshots I strongly suspect that even a user from XFCE 3.x would find the very latest version to be a drop-in replacement. And it does have decent quality, which combined with its general "get out of the way and let me work" vibe and 20 years of continuity makes it easily one of the best options available.