The thing is many people who would not vote by going out or cannot be bothered suddenly vote because the mail in process is easier than standing in long lines depending on where you live. The politics of the past few years was so polarizing we saw an uptick in voter participation on BOTH sides if you look at votes received by Trump and Biden. Both received record number of votes.
I think many people stayed away in 2016 thinking erroneously it was a slam dunk (not sure why) and they were not going to make the same mistake again. Trump won MI, WI and PA by very slim margins which many people forget was the actual margin of victory in the 2016 election.
That means both sides were going to mobilize their bases in a major way. I think COVID and the scare made a lot of people who vote normally simply switch to mail in. I went in person and it was EMPTY. I am pretty sure most of the people that usually go to my polling place simply decided to mail it in. Imagine that happening in districts all over the country.
the problem is the system is not set up for mail in voting in such a large scale. In every election votes are still counted after Election Day due to mail in votes but the numbers are smaller. The next big election is in 2022 and COVID should NOT be a factor and neither should mail in voting at all.
In the long run mail in votes will still be mainly for overseas/military/disabled etc...
2022 will be a large GOP mobilization and with COVID going to be 2 years behind us at that point...I think it will be a more normal election.