You got it exactly right up to the last sentence. Yes it is exactly the same amount of work to figure out how to make a long distance phone call or schedule a meeting.
Except for most people it is almost never a problem because most people aren’t scheduling phone calls or meetings with unfamiliar locations.
The benefit is obvious: things that happen at the same time have the same number associated with them. Just like we currently coordinate dates using a shared calendar, we coordinate times. And that has a local benefit as well. Aside from no more daylight savings time which we can achieve using other means, we also know when world events happen as they are being reported, we know when certain things take effect for countries that span multiple timezones now, we know the exact difference between any two given time points without asking “but where are they happening?”
Again, imagine that we had unified time and some country somewhere said “hey we are using the same format but doing our own variable offset from it”. That would be absolutely asinine. We are used to timezones and think they benefit us because of that familiarity. In reality we have had variable time even in the same timezones before and it was universally a bad idea that was shut down soon as people started traveling.
Oh and consider that due to cultural differences your example of scheduling meetings is already flawed: do you know if Tokyo wraps up the workday at 4pm or 6pm? Is there an hour or two hour lunch break in Barcelona? When do people actually come to work in Rome? Are banks open on Saturdays in Baghdad? Timezones do not solve these issues whatsoever so you still need to do the lookup of “when are people actually around” when scheduling a meeting outside of your culture.
1) Daylight saving time has absolutely nothing to do with this, and in any case a lot of people, me included, are of the opinion that DST is a Bad Thing and should be abolished.
2) If you want to let everybody in the world know when something specific happens (e.g. the launch of the first manned Mars mission), then you specify the time in UTC. But.. that's what we do already. Or at least those with some sense. There's still no need to force Joe Nobody to get to work at 14:00 (assuming a single world time zone) when the sun rises.. there's no advantage for people elsewhere in the world. You yourself argued that "most people aren't scheduling phone calls or meetings with unfamiliar locations". So, what's the advantage here?
3) I happen to use a calendar which is coordinated with others - it happens automatically, when they schedule something. And it has zero problem with the time - it's not just date. There's zero to gain by using UTC as localtime everywhere. That does nothing for the calendar.
4) Cultural differences in work hours: They absolutely exist. So what would having everybody use UTC help with? You still have to know those differences. It's even worse then, because if my stepson tells me "I have to work to 10AM" then I know it's late, for him. I know it instantly. If he had said "..have to work to 13 UTC" then I'm lost. I don't immediately see that he's actually working very late. I'll have to think "let's see.. he's in Tokyo, so that would be, hmm, this many hours difference from here.. is there an issue here? Oh wait, "you're working really late, will you be you okay?"
5) And that statement ".. most people aren't scheduling phone calls or meetings with unfamiliar locations". That's false. We're not in the 19th or even the 20th century anymore. The internet is a thing, and tons of people are in daily contact with people from all over the world. Just a moment ago I got a message from an airbnb guest, for example.
4) Argh, I meant 10pm or it doesn't make sense - I shouldn't try to use English time-units, I should stick to the 24-hour system I know. I always mess up that.
Except for most people it is almost never a problem because most people aren’t scheduling phone calls or meetings with unfamiliar locations.
The benefit is obvious: things that happen at the same time have the same number associated with them. Just like we currently coordinate dates using a shared calendar, we coordinate times. And that has a local benefit as well. Aside from no more daylight savings time which we can achieve using other means, we also know when world events happen as they are being reported, we know when certain things take effect for countries that span multiple timezones now, we know the exact difference between any two given time points without asking “but where are they happening?”
Again, imagine that we had unified time and some country somewhere said “hey we are using the same format but doing our own variable offset from it”. That would be absolutely asinine. We are used to timezones and think they benefit us because of that familiarity. In reality we have had variable time even in the same timezones before and it was universally a bad idea that was shut down soon as people started traveling.
Oh and consider that due to cultural differences your example of scheduling meetings is already flawed: do you know if Tokyo wraps up the workday at 4pm or 6pm? Is there an hour or two hour lunch break in Barcelona? When do people actually come to work in Rome? Are banks open on Saturdays in Baghdad? Timezones do not solve these issues whatsoever so you still need to do the lookup of “when are people actually around” when scheduling a meeting outside of your culture.