Actual good UI/UX design isn't trivial and it tends to require a tight feedback loop between testers, designers, implementers, and users.
A lot of FOSS simply doesn't have the resources to do that.
I'm not disagreeing with your basic take, but I think this part is a little more subtle.
I'd argue that 80% of users (by raw user count) do want roughly the same 20% of functionality, most of the time.
The problem in FOSS is that average user in the FOSS ecosystem is not remotely close to the profile of that 80%. The average FOSS user is part of the 1% of power users. They actively want something different and don't even understand the mindset of the other 80% of users.
When someone comes along to a FOSS project and honestly tries to rebuild it for the 80% of users, they often end up getting a lot of hate from the established FOSS community because they just have totally different needs. It's like they don't even speak the same language.
It was something like:
- almost everybody only uses about 20% of the features of Word
- everybody's 20% is different, but
- ~80% of the 20% is common to most users.
- on the other hand, the remaining 20% of the 20% is widely distributed and covers basically all of the product.
So if you made a version of Word with 16% of its feature set you would almost make everybody happy. But really, nobody would be happy. There's no small feature set that makes most people happy.