But designers and developed don't support one UI, or even two. They support anywhere between three (desktop, mobile, tablet) and effectively infinite (all resolutions between and around those + dark modes + apps which use the same data + all different browsers). UIs can't be very good one on platform without either making each platform's UI separate or sacrificing functionality on other platforms.

New GitHub looks alright on low resolutions, but looks kinda awful on big screens. This is the opposite problem old GitHub had.