I think that your views on the design are more than a little harsh. It has some elements that can be improved and there are a number of constructive discussions already in flight about it: e.g.
- A thread discussing more general UI/UX concepts after a UI change).
- A detailed discussion with informative posts by @uxamanda and @andrejilderda
- CSS improvements
I think your assertion that "only limited people will use it" with the current design is demonstrably wrong.
The mobile application I do agree is one of the weaker aspects currently but it is still very much functional (can't say I'm one for editing detailed markdown notes on a phone, I'll jot stuff down then edit it properly once back on a computer).
There is already a lot you can do in regards to customisation:
- @andrejilderda's macOS theme for a fairly comprehensive re-skin of the entire app
- Customisation wiki
- Share your CSS thread
Yes there is probably going to be more to come, Joplin has come on a huge amount within the last year alone and is very actively developed with lots of community feedback and engagement.
Feel free to air any specific examples of criticism - thats what things like the forum are for - rather than "can't get used to it" or "looks very slow and bad" which doesn't really help anyone.