> even eg scheduled video calls take place in physical places, so in practice future events of any kind are best stored in localtime with the relevant user’s timezone attached

I was in London when I scheduled a video call with someone in New York. When we actually took the call, I was in Spain and he was in Chicago.

I can’t think of any timezone that could have been attached to that meeting to make it useful at the time of creation. I absolutely expect my machine to be able to tell me what the local time of the meeting is when its timezone changes.