causes the question “What time is it there?” to be useless/unanswerable
That is a feature, it removes one thing to worry about.
necessitates significant changes to the way in which normal people talk about time
Yes, I think this is the biggest argument against. It would take a long time to get used to.
convolutes timetables, where present
How?
means “days” are no longer the same as “days”
Same as point 2.
complicates both secular and religious law
How?
is a staggering inconvenience for a minimum of five billion people
How?
makes it near-impossible to reason about time in other parts of the world
How? In my opinion it makes it easier.
does not mean everybody gets up at the same time, goes to work at the same time, or goes to bed at the same time
Yes. This is true.
is not simpler at all
Of course it is simpler. You have just removed a huge source of complexity. It still isn’t simple because people will still live their life at different times. But it is simpler.