I mean, everything is subject to abuse. The relevant factor is the likelihood and difficulty of such abuse, isn't it? And I don't really see how using the stamped date as an authority reduces either or how not doing so increases either. The way you'd commit abuse changes, sure, but doesn't make abuse any easier/harder in my opinion, just different.

Intentionally delaying a mailing is already a crime in itself. And if you already have the power to shut down a mailing hub for a day or two without abyone noticing, I don't see how it would be more difficult to change the date in the stamping machine.

I have no doubt that, when the Nazis come to power here as well, they will use all measures to suppress free voting. We of all people know how that works. But in that case, using the stamping date for mail-in voting won't stop them anyway, as that's a simple law, that can simply be changed by the people in power. Case in point: This very article about changing the postmark application rules. It seems, they didn't even have to change a law for that, just some USPS internal guidelines.

how is what different than whos current system?

The US system, using the date of the stamp instead of the arrival date. Neither system is safe from abuse. It's just different.

Mail-in voting is inherently more error prone than voting in person. That's why it's important to make in person voting as accessible as possible. Voting on a Sunday, having plentiful accessible polling stations without significant waiting time surely helps, and is a reason why the share of voting by mail is traditionally way lower here in Germany than in the US. I never had to wait for more than 2 minutes at a polling station here in Germany.

Have a nice new year's eve and guten Rutsch, as we say here.