So it isn't a mail voting problem its a voter purge problem. Ok. We have that technology, but let's do that. Let's have a big discussion (or not so big here in Indiana) about what it takes to remove someone from the lists of people allowed to vote.
But, even in California, those are going to REGISTERED voters. And if we aren't sure about who is actually filling in the ballots, then: A) that's not much different than current absentee ballot processes; 2) both sides have Get Out The Vote initiatives to "help" people fill out the ballots.
Purging isn't going to happen. Some groups fight that. They say it's "disenfranchising" because it could purge someone who should not be. Like I said, I have less of a problem with no-excuse absentee voting systems, because if you have to request the ballot, they can take reasonable steps to make sure you're the person identified on the registration, and that you still live at the address of registration. And then, if you get a ballot in the mail, it's because you asked for it, and not because some dead person who lived at that address had registered.