Mail-in ballots have existed for decades in elections all across the country and no one has questioned their legitimacy. If you accept the results of elections that have occurred prior to today, then there is no basis for the argument that mail-in ballots are illegitimate all of a sudden.
The problem isn't their legitimacy, it's that there have been numerous stories about one or very few persons affecting hundreds or thousands of votes, basically like an amplification attack on democracy. When mail-in ballots are only a small percentage of the overall votes it's not as large of an issue. What Trump asserted was simply that we'll see a sharp increase in the number of fraudulent votes because the attack surface is going to be exponentially larger.
Trump's assertion is based on the notion that mail-in-ballots see higher rates of fraud than in-person. It's not difficult to see why that would be the case, but I will concede I don't have first hand numbers.
The insidious thing about this situation is that there is now a lot of anger on both sides. If cooler heads had waited a bit longer, collected real data on the rates of voter fraud, actually addressed Trump's concerns about stolen/forged ballots rather than calling him a liar and linking to puff pieces from two of his biggest and unfairest outlets, we would stand a better chance at resolving this amicably.
If you were told that gerrymandering has a demonstrably larger effect on an election's legitimacy than mail-in voting, would you be as vocal against gerrymandering as you are against mail-in voting? If it is really about election legitimacy to you, isn't that what you should do?
