Take some place currently having UTC+12 time zone, say Marshall Islands. Midnight by UTC, the moment date changes, is exactly noon there. So how should people there talk about time? There is no “Tuesday the 15th of May” there, because every day is one part one date, other part another date

So yeah. For computers and programmers “whole planet lives in UTC” might look like a boon (for a time I myself wished for it), but only until they start facing other, more twisted problems