The problems with wide-scale mail-in voting
Having served as an election judge, I can see several key issues with ubiquitous national voting by mail. Perhaps most important, registered voter data bases are not an accurate source for mailing out ballots. There are reports of districts with as much as 20% more registered voters than there are residents, many due to relocating or deceased voters. The accuracy of this information is critical, particularly in close elections.
Another concern is sufficient capacity to process the ballots, not just postal workers, but ballot processing personnel. Earlier voting could help, but campaigning and debates would have to start even earlier to address the peak demands.
Voting in person provides a number of safeguards. It ensures that an actual person is voting, is a valid registered voter, validates their signature, and ensures that they receive a proper ballot. It also provides for on-site registration, processing name and address changes, and redirecting voters to proper polling places to ensure they can vote.
Absentee balloting addresses some, but not all, of these issues. To ensure truly fair voting by mail, a concerted effort would need to be undertaken including scrubbing registered voter databases to ensure their accuracy, automating ballot processing with technology like coding and scanning ballots, for example, to facilitate secure automatic validation, and proper staffing to ensure timely counting.
We are not ready for ubiquitous mail-in voting in this election. For now, in person and absentee voting are the best approach.