Because time in the real world uses leap seconds. The time businesses open and close. The times reported in news articles. And so on. What facebook wants is for all of them to stop using leap seconds so facebook’s infrastructure is simpler
https://www.cl.cam.ac.uk/~mgk25/time/leap/
Time in the real world can use leap hours, the next of which will be in approximately 3000 years.
This is all quite hypothetical as it’s hard to predict whether anything like our current technological civilization will exist thousands of years from now, but even if it does, a simple mechanism exists to avoid problems (just have each jurisdiction change their local time when they decide they want to).
I can’t see any downsides for literally anyone from this proposal, other than the insignificant downside to the British that they will lose the prestige of being the place that global standard time is based on.
> lose the prestige of being the place that global standard time is based on.
You joke, but that's actually a source of some of the opposition. :(
It infuriates me that the BBC World Service insists on announcing the time as "<something> GMT", pronounced in a smug tone of voice. The GMT timescale no longer exists; nobody broadcasts it. What the announcers are broadcasting is the BBC's ignorance. I find it embarrassing and jingoistic.
They've also adopted an idiosyncratic way of pronouncing the time itself: "The time is Four, GMT". Everyone else says "Four O'clock", or "four hundred hours" or "four AM". The Beeb are almost completely immune to complaints; they've outsourced their complaints department, and the contractor's brief is to make sure no complaints reach programme makers.