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.

>I figure in a few centuries, a lot of communities will be massive buildings without any sun exposure anyway.

I would be surprised if that turns out to be the case. People need sunlight to be healthy, and while you could certainly create artificial sunlight it would still cost resources, and there's a big benefit to having most people awake during roughly the same hours.

There are massive downsides to having everyone on the same schedule, too. Most infrastructure has to be a lot bigger to accommodate the peak loads. Roads are plagued by "rush hour," call centers have "longer than expected hold times," and coffee shops have to decide whether to ask staff to work short shifts or pay them for periods when they are not needed very much. These things reduce productivity and even kill people.