The difference is that you’re not changing how time is kept. Countries can change their timezone offsets right now to screwy things like +12:45 and it changes how time is recorded and stored. If we switch to UTC, a country can just declare their official hours are shifting and nobody has to fundamentally change how clocks work.
IMO people would figure it out and life would go on. Yes, lots of people would have the calendar date advance in the middle of the day but that’s fine, we’d get used to it. People wouldn’t work 9-5 jobs, but we’d come up with different terminology.
I don’t really see the argument about people waking up at different times. Yeah, some people would wake up at 02:00 and some at 16:00, but when someone says they wake up at 02:00, there’s 0 confusion about when that is. You’d have to know when someone is awake to do an international call, but you have to do that anyways.
I’m pretty meh on the arguments there. It would be different, but we’d adapt. And we’d fix a lot bunch of problems.