It's not like this is some new method of voting. We already have mail-in voting in California. This just expands it. Oregon has been entirely vote-by-mail since the 1980s.
There is no fraud. You know why there's no fraud? Because it's a crime, and unless you attempted an absolutely massive fraud, which would be just extraordinarily easy to detect, there would be scant little point in taking the risk. Who would risk five years in prison for something that has very little expected impact?
There have been plenty of investigations into vote-by-mail in Oregon, and no fraud has been found.
This is all just a false narrative to suppress voting among people less disposed to stand in line and do it - the poor.