There's also the inconvenient fact that all of the USPS anti-fraud protections are mostly retroactive and involve special sections of US federal law with big penalties for abusing the mail or committing fraud through it—after the fact.
The US Postal Inspection Service (the mail has their own feds, did you know?) would take well beyond the window of the election results being declared if some criminal decided to take the same blatant measures to defraud an election by mail as they did to defraud the electronic voting systems the last election, which were fraught with actual issues of undefined electronic voting integrity (which, of course, fails unsafe, because you're not going to re-do an election when the numbers don't match). It's entirely plausible that the last US presidential election was stolen electronically and the perpetrators simply got away with it because computer security sucks.[1]
Ultimately, voting is a hard problem, and it's not really solved presently (in person, electronically, or by mail) from a technical/attack perspective. There are so many ways to straightforwardly attack the system right now if you're willing to risk a billion years in jail if you're detected, most of them related to the ways that electronic votes are collected[2] or the ways that votes are transmitted to central tabulation.[3] These systems fail unsafe, even when discrepancies are detected.
(My own personal theory about this is that if election results take too long to become "official", people will lose faith in the process itself, so getting to "done" is in general more important to most involved than getting to "perfectly accurate".)
Vote-by-mail has a different, but also still large and present, attack surface, especially if you (as above) are willing to do Big Deal Federal Crimes Involving The Postal Service to subvert it, which I believe is entirely on the table for the highest office in the land.
I don't care for the president or his tweets, but to call this one in particular anti-democratic might be a stretch, too. The US presidential elections simply are not secure in any meaningful way right now, and switching to mail won't fix (or worsen) that. (I know it's not a popular thing to point out that the US election security emperor has no clothes, but it's unfortunately true.)
It's not antidemocratic to say that it's fairly likely that elections have been, and will continue to be, successfully hacked. (The tweet in question is, of course, using the concept as a weapon to further a specific antidemocratic agenda, but that is another matter entirely.)
[1]: https://www.theverge.com/2018/7/17/17582818/election-softwar...
[2]: https://www.newyorker.com/news/news-desk/how-voting-machine-...
[3]: https://time.com/4599886/detroit-voting-machine-failures-wer...
Finally,
> might as well be state run if it does not have the freedom to structrue the presentation of platform participation.
Totally with you on that one.