I think it needs to be said here: cross-timezone scheduling and datetime math are extremely rare relative to people's daily experience. It may not be obvious to us here, because we're much more likely to be dealing with the unusual case - building distributed systems, having international users/customers, working for multinational companies or cooperating with companies from other countries, etc. Almost nobody ever does that, and it makes no sense to make everyone do extra mental arithmetic when thinking or talking about other places, just for the sake of a minority of white-collar workers who can't be arsed to add a second column to their Outlook calendar, or double-check when they have a Zoom meeting.

Getting rid of timezones would make most of everyday matters location-dependent. Timezones allow us to use the same numbering of times of day regardless of where any of us lives. 08:00 is morning. 22:00 is night. 12:00-14:00 is around when people have lunch. Etc. We can talk about our days in all kinds of ways (movies, plays, songs, novels, guides, etc.), retaining a precise numerical scale (vs. much less useful terms like "morning" or "afternoon"), and everyone everywhere can relate. Getting rid of the timezones makes this impossible.

Note the difference between talking about daily experience with people around the world, vs. doing things in sync with people around the world. Timezones make the former trivial, and the latter easy. Removing timezones makes the latter trivial, but the former hard. Hard enough that people would quickly recreate timezones for convenience.