A secret ballot and chain of custody both rely on the idea that the ballot is verifiably given to the voter, filled out in secret, and never leaves the control of the voter before being submitted in a way that removes the connection to that voter.
Literally all of this is invalidated by a mail in voting system. You cannot be sure the proper person got the ballot, you cannot be sure the person filled it out without it being seen by others or coerced, and you cannot be sure the ballot wasn’t intercepted somewhere along the line and modified. The entire system hinges upon a signature verification process which is automated and obviously needs to avoid false positives more than false negatives, given the risk of disfranchisement. We should expect the signature check to be a poor mechanism to block hijacked ballots, anyone with a bit of sense can see why we should expect it to allow most signatures through.
What we should expect under a mass mail in voting system is fairly obvious, given the means, motive, and opportunities it uniquely affords people in highly contested elections.
It has been interesting to watch smart people (not you) who were warning about these risks years ago be forced to stop agreeing with them because of the political implications that would likely come if we had verified examples of these weaknesses being exploited.