To be honest as a user you don't see the difference with 2000s-era featurephones: the UX is the same (same kind of menus, same D-pad navigation, same slowness here and there ...). You don't have any idea it's all a bunch of javascript files; it's not like you have ad-ridden bloated sluggish websites.

As a user I don't believe it matters that much what OS or what language you're using as long as it's stable and functional. KaiOS definitely fits the bill