I agree Dwarf Fortress is quite hard to learn and the UI plays a large part of that as it's completely keyboard based and requires a lot of upfront effort to learn, but once you do you become much faster than you would otherwise (much like text editors).
People often complain about Dwarf Fortress's graphics in the same vain and breath as the UI, but I think these are parts of the charm and instead of being weaknesses they are leveraged as strengths.
For instance, the lack of fidelity of the game allows any new character to be added in 2 seconds, yet Rimworld needs a considerable amount of time and effort developing each texture. To an extent I think the 'horrible' user experience cannot be divorced from Dwarf Fortress. Losing is fun, after all.
I think what you consider a 'horrible' user experience cannot be divorced from what we know as Dwarf Fortress today. Losing is fun after all. I love Rimworld and Dwarf Fortress, but they occupy different spaces within a similar genre in my mind.