That prevents you from doing the correlation after the vote has been counted but not before.

To do the correlation in person you would have to do it in the polling place, on election day, in front of election monitors and the voter, during the 30 seconds between when the person fills out their ballot and when they drop it into the box.

If your name is on the envelope, now it can be done anywhere between your mailbox and the polling place during the period of days or weeks between when you put it in your mailbox and when it gets counted. Wouldn't we then need independent election monitors guarding the incoming mailboxes 24/7 for several weeks?

in Oregon the ballot goes in an envelope with no identification, and that envelope goes inside the envelope with identification.

The outermost envelope is verified and opened, and the innermost envelope is collected with all of the others in a different area for counting.

There is an implied identification that is effective. Some neighborhoods vote one way, and other neighborhoods vote the other way. Postal workers can purposely misplace/delay/destroy the votes for a whole neighborhood.

I think the risk/reward would make this not worth it. You can check to see if your ballot was received. If a bunch of ballots are determined to be lost from the same postal route one assumes the postal inspectors would be having a word with that mail carrier. Tampering with the mail is a pretty serious crime, ballot or not, so there's a strong disincentive to try it.

My town has a bunch of dropboxes which are picked up by election volunteers if you still don't trust the mail. An election volunteer could "lose" a box of ballots but that seems like it would be pretty easily detectable as well.

Since mail tampering is a federal crime maybe Trump could pledge to pardon every mail carrier who throws away ballots from likely Biden voters. Seems pretty unlikely, but I don't know how to counter that.