There are 2 use cases for UTC:

1. People who don't leave their regional time zone very often.

2. Those that do.

The first group of people would have a week where they aren't used to getting up at 1:00 or eating lunch at 23:00. But that won't last long, because the schedule maintains consistency and people would just shift their hours accordingly.

The second group can now plan a flight, report the time they are landing to a colleague, not have to worry about time zone conversions, schedule a meeting 2 hours after that and know exactly how long in the future that is without worrying. An international conference call can just be scheduled for 5:00, and you don't need to mix time zones to figure out when that is. Two business associates can say "I'm available from 12 - 2, and you are available from 1 - 3, but since we use UTC we don't need to actually change those times to make them match, we just know the overlap".

That second use case is the one where current time systems are woefully inadequate, but because the majority of people persist in the first kind of time system, there isn't enough pressure to change to benefit the second. It requires coordinated effort, that a tiny wink of time where people have to adjust to a shift in the hour they get up or eat in the day is much worth the benefits of not having international commerce and business have to constantly adjust their times and do mental gymnastics to keep cross-time-boundary timekeeping in check.

Again, not in Spain, where 16:00 is siesta time for many if not most shops and a closed period for restaurants. You seem to expect what you're accustomed to, all over the world. But it doesn't work that way, even for seemingly simple cases. And it isn't just Spain--for example, Hong Kong's electronic trading closes from 12:00 to 13:00. You'd have thought you could trade at mid-day on any exchange, but you can't!

I'm going to argue that if we did a global switch to UTC, current trends are already leading to border time zone regions migrating their schedules closer together (the American "day" has been steadily getting later in the actual day over the last century, and I'd argue that is in part due to a business-wise synching of times with west Europe).

We are caring less and less about when the sun is out. I figure in a few centuries, a lot of communities will be massive buildings without any sun exposure anyway. We will be flying through space and metrics like the height of the sun over the horizon no longer matter. Time isn't something best represented as arbitrary numbers across datelines, it is best represented as a consistent, constantly incrementing value. The fact the sun rises and sets in different parts of the world at different times isn't an excuse to keep the time system reflecting a giant ball of plasma that less and less drives our daily activities.