mass mail-in voting, which has the same downsides as online voting

What exactly do you mean, the same downsides?

Mail ballots require physical presence to tamper with, so if you have armed guards + security cameras watching the drop box, most tampering threats can be prevented, at the same time, no one knows who the voter really voted for since envelopes are sealed.

In contrast, internet connections are really easy to tamper with. Most people just blindly click past https warnings, and thats just the most basic attacks, there are more sophisticated attacks. Not to mention, you dont even need physical presense. Anyone, anywhere around the world can hack an online vote.