The POTUS voted by mail. WTH are you talking about? Voting is a State issue and voting laws are already in place. If a State decides they want to change their voting laws then the legislature can debate, vote and change. What you can't do is unilatererally change voting requirements and send out mail-in ballots contrary to State laws because of some pandemic exception. There needs to be protections in place to ensure that voter fraud doesn't occur because the integrity of our elections is paramount.
https://www.heritage.org/election-integrity/commentary/potential-fraud-why-mail-elections-should-be-dead-letter
"Going entirely to by-mail elections would unwisely endanger the security and integrity of the election process, particularly if officials automatically mail absentee ballots to all registered voters without a signed, authenticated request from each voter.
Voter registration rolls are notoriously inaccurate and out of date, containing the names of voters who are deceased, have moved, or otherwise have become ineligible."
I believe CA found 1.5M voters who either died, moved or became ineligible to vote on the registers. Now imagine that in a swing State where the difference in votes is 100,000 or less.