I'm sure voter fraud exists. I've heard stories of some that was done in my county some years ago. I know it goes on. But is it widespread? Well, no one has definitively proven that it is.
As far as I can tell, floating the possibility of massive voter fraud is being done in preparation for if/when Trump loses. I'm reasonably sure that Trump will contest the results if he loses and he is setting up the possibility of voter fraud as one of his reasons for contesting.
By the way, I applied for a mail-in ballot and was just told I was approved. In this state one has to apply for a mail-in ballot and the applicant had better have a good reason for not voting in person. With COVID, I believe these applications are being routinely approved for old people with health conditions like yours truly. In any event, there is no blanket mailing of ballots to every Tom, Dick and Harry.
Is voting by mail completely safe from fraud? No, not 100%. But I would not want to have it eliminated entirely. In my case, my polling place was moved to the basement of a church with dubious ventilation. It's way too risky for me to be packed into that small space with a bunch of strangers who potentially could be infected. Not going to take the risk. The same applies to others who are older and are particularly at risk.