The issue with that is that it is unfair to the lower-ranked skaters who may then miss out on an Olympic spot altogether if big feds are allowed to pick their best result.
Remember, there are only 30 spots in singles, and even less in pairs and dance.
In practice, I don't think it would have a big impact. For Ladies, KAZ and CAN might be overrepresented if 2019 results are used, but Russia and Japan would likely get three again in 2021, US and South Korea probably two also. So that's maybe three extra spots (assuming KAZ can even send three skaters with minimum scores), but the benefit is that it's a somewhat fair compromise given the unprecedented circumstances. It's pretty clear that this is going to be a rough season anyway, given how shaky a lot of last season's top skaters have looked after the extended break.