The only reason that millions no longer trust mail-in, and more generally, the only reason anyone thinks there is widespread cheating & rigging of elections - is because a pathological liar who can't stand losing said so.
He said that 2020 was rigged, and that there was "massive fraud" in 2016 (only because his vanity couldn't handle losing the popular vote), and called 2012 a "sham."
Trump has basically single-handedly eroded trust in the integrity of our elections for millions of Americans. It will be his most prominent legacy.