Put aside for now the question of whether the presidential election was "rigged" against Donald Trump. Put aside the partisan pronouncements about whether Trump or Joe Biden is the legitimately elected president.
Let's instead take an objective, nonpartisan look at whether the election was was good enough to become an acceptable template for elections to come.
I think not. Because the chain of custody of too many ballots was broken
The chain custody of ballots is as important as the chain of custody in criminal cases. It means that nobody or nothing can intervene between the voter who casts the ballot and its counting. Allowing middlemen or third parties to get their hands on the ballots is an invitation to fraud.
When voters cast their ballots in person at a polling place, the chain of custody is in the hands of the voters themselves. They mark the ballot and they feed it into the machine that counts the votes. The count often is backed up by a paper record
Consider now the route of vote-by-mail ballots. Blank ballots are mailed out to millions of potential voters, including some who no longer are eligible go vote in a particular jurisdiction and some who have died. (That's why it is so important for voting rolls to be regularly updated, to weed out ineligible voters. To pretend that updating the voting rolls is a racist is a lie.)
Anyone who gets his hands on the blank mailed ballot can cast the vote in someone else's name. Especially when their signatures are not verified as they are in a polling place. (Voter IDs would be an even better guarantee of honest elections. Pretending that requiring a voter ID is a racist plot is another lie.)
Verifying the signatures of mail ballots can delay the counting beyond legally required deadlines, creating various opportunities for fraud. For example, illegally marked ballots can be trotted out in the dark of night or "discovered" when it was determined that a "few extra votes" were needed to boost a preferred candidate.
But even legitametly marked ballots are not safe. They can be picked up by people who have a vested interest in an election's outcome and trashed when it becomes clear that they would help give an opponent a victory.
Unattended drop boxes for completed ballots are a weak link in the chain of custody.
There are so many ways that elections can be fixed when the chain of custody is broken there isn't enough room here to list them all. Suffice to say that if we want honest elections, vote-by-mail ballots are not the best way to get them.
Here's a chance for real bi-partisan cooperation--if Democrats can put aside their insistence that any move to ensure clean elections is a racist plot to suppress the minority vote and Republicans can look beyond their deep belief that the election was rigged against Trump.
If I had my way, I'd end vote-by-mail schemes of most sorts, especially when a state is flooded with millions of unrequested empty ballots. If a person can't be around on election day, in-person early voting is a viable option. For those who can't travel to a polling place, requested absentee ballots are available.
But that won't get done, so legislatures will have to figure out a way to improve the legitimacy of vote-by-mail ballots. Doing nothing or extending the mail ballot system, with its built-in errors, to more jurisdictions simply is not acceptable if we're to avoid he shambles that we've had to live through with this election.
Failing to safeguard ballots in the next election is the real and most obvious threat to democracy.