Man City for the domestic treble again in 2021

[Rogin the Armchair fan]

They'll have no European distractions....

Ah, snake or admin6, hobbes beat me to it so delete this one please!!

[Tony C]

It's one way of easing the fixture overload.

[Sporting]

Why aren't they being thrown out of this season's competition?

[Janik]

Because competition authorities don't like that sort of disruption midway through an event, or they need to reclaim payments already made that it would ential. Much cleaner to not allow them to start in two future seasons than to remove them from this one and the next. Or indeed retrospectively delete their results from competitions since the offence happened.
And any such immediate ban would have likely been stayed pending appeals anyway. So, again, cleaner and more straightforward to make it for events yet to start in the near future than ones already in progress.