That's highly debated. My state has voted by mail for at least a decade and I'm not aware of any problems.

People will debate whether mail in voting is good or bad, fraudulent or not, etc. That's fine, let them debate, legislate, appeal to courts, etc.

But besides all that: There is simply no scenario where allowing the USPS to half-fail improves the situation.

Even if the USPS was running perfectly, this election would still have unusually low trust, because of the pandemic and because of our especially toxic political climate. Adding a hobbled USPS to the mix makes things much worse.

Lots of people are saying Trump is trying to "muddy the waters" of the election. I suppose Trumps motivation may be debated, but I think the result is clear: A half functioning USPS will "muddy the waters" of the election result. Does anyone disagree that this will be the result? Even if (being charitable) that was not the intention.

Please provide credible evidence for this claim.

But only for roughly 1/2 of people. For the other half, voting in person would disenfranchise some demographics.