Mail-in ballots don't generally increase the instances of election fraud. All of the "what ifs" have been investigated in the states that have had general-populace mail-in for years and been found not to occur in either (a) numbers that sway the election or (b) numbers distinguishable from in-person fraud (which is usually of the form "person not eligible to vote, but voting office screwed up and granted them a card" or "person moved and failed to notify election boards of the relocation; voted in the wrong district").
Every election has at least one story of rampant incompetence and/or outright fraud where one person is able to alter hundreds to thousands of votes. For elections where it can literally be 50.2% to 49.8%, it can make all the difference.
Rampant incompetence isn't the same as fraud, and one would still have to make the case that we lose hundreds of envelopes more often than we have an entire data-store go missing (or, for that matter, than we have a fleet of digital voting machines crash and deny access to the polls to face-to-face voters for hours).
