I'm not sure any of these answers are the best, though they could possibly be.
Jakob I guess I completely agree with, can get healthy.
The older ones while yes it gives more time to build back fitness, it's also more time to get hurt being old.
Likely there are college kids or kids of that age who maybe will be hitting their prime in that season if the games move to 2021.
Side note: No chance they happen this summer. Not sure why the IOC is even still entertaining it.