The idea that a sitting President who seems to be losing popularity and one that polls show is going to lose the election might use underhand tactics to delay an election is not far fetched. I am not saying that Trump will do that or attempt it, I am just saying that many strange things happen in politics. If as you stated there is too much to lose for the US and its citizens by postponing this election because of various protocols and red tape then it must go ahead on time.
Whether we are talking about mail voters or electronic voting machines in voting stations, there are many ways to manipulate the system and participate in vote rigging. Again, not saying that will happen in November 2020 but accusations of vote rigging are rife all over the world recently. Trump seems to be saying there is a risk of voter fraud as a result of mail voting but is that even a valid reason?
There is an arguement that an election that occurs on any date other than November 3 would be invalid without a prior act of congress. Trump even trying to delay the election would cause a major clusterfuck within the US government.
This is not to say that having a mail-in election would involve equal, or possibly worse fuckery. A mail-in election is almost guaranteed to produce results that will be questioned by anyone not on the winning side. Traditionally, absentee ballots number less than the margin of victory, so screwing them up will not affect the outcome of the election. It is not uncommon for mail to be delayed or lost, and a small percentage of votes being lost, or "lost" could easily flip the results of an election.
Mail-in voting also raises the risk of fraud due to forged ballots. The design of a ballot is going to be public because they will be distributed to the public, therefore anyone could possibly print an arbitrary number of ballots, and mail them in envelopes similar to those used to return ballots to election officials (also public).
The expiration of the enhanced UI benefits will make the lockdowns much less popular, and if lockdows are lifted, it will be more difficult to allow everyone to vote via mail.
We have to remember that vote rigging is more possible in local elections where the only determining factor to determine the outcome of a race IS the popular vote. It's harder to do in the electoral college b/c of the fact that you'd have to know beforehand where the best states are to attempt to rig. Even with just knowing in broad strokes the swing states, it's still hard to get another 30k or so votes into a state.
In a regular local election which only relies on the popular vote, it's a bit easier. But with the EC it's not as easy -- b/c a rigged vote in Cali does not change the fate of the vote in another state.
But yeah, a sitting President obviously shouldn't be able to delay when their election happens. That's a clear conflict of interest and it's good that the founding father saw that a decision like that should be left to the Congress.