Irrelevant appeal to authority. Just because they are allowed doesn't mean we should accept and allow them. The military vote is an exception to voting in person. What's bad--really bad--is letting everyone and anyone vote by mail.
If you send a the registrar a photocopy of your ID and a signature I and they send you a ballot . I don't have an issue with you casting a ballot.

But sending out a ballot to the address of every voter on the voter rolls...Oh hell no. To many people move and no longer live at the address on the rolls. There are multiple cases of people getting multiple ballots just sent to their address. Anyone that would vote for brain dead Joe Biden would be dishonest enough to cast multiple votes.
Not in this case. Other countries have tried all-mail-in ballots and elections and after a few years rejected them as unsecure, prone to fraud, and unsafe in the sense of the outcome.

...about mail-in vote fraud, there are numerous examples of vote fraud and vote buying with mail-in ballots in the United States and across the world. Indeed, concerns over vote fraud and vote buying with mail-in ballots causes the vast majority of countries to ban mail-in voting unless the citizen is living abroad.
There are fraud problems with mail-in absentee ballots but the problems with universal mail-in ballots are much more significant

The absence of proof isn't proof of absence.

For example, during the 2020 election, I got a text message from a person at a group called Mi Familia Vota. It's a Leftist 501(c)3 nonprofit voter advocacy group. The text was for someone other than me (named), and the person from that organization wanted to know if I needed help filling out my ballot and getting it delivered. Both of these actions are illegal in Arizona, but there it was in writing. Yes, I did report it to the state, but never heard anything back on it.

While that's anecdotal it was proof to me that voter fraud was going on and likely not being detected or acted on by the authorities. So, yes, I can personally point to fraud in our elections having actually experienced an attempt at it.
Mi Familia Vota is obviously a hispanic nonprofit offering assistance to those who struggle with English. Fail.

Other countries? Appeal to authority. Fraud is minuscule here. With the incredible amount of counts, recounts, audits, investigations, the absence of proof IS proof of absence.

Mail in votes. Here to stay. And continues to expand.
Leading to "massive voter fraud"?

What evidence is there it has led to any significant voter fraud? In order to have an informed opinion on that question you need to know how the system works. Do you?
How would you or I know? The whole system using all-mail-in ballots over weeks and weeks gives plenty of time to perpetrate massive fraud that is for all intents, undetectable.