It's better to worry about these things before they are problems. I don't know if it's happened in the US but it is absolutely a very common attack against voting systems, and any voting system not resistant to it should be considered suspect.

Voter suppression is an actual real world problem that happens today, not a hypothetical problem like the one you are worried about. Allowing voting by mail is an improvement to that real world problem as it reduces the barriers to voting for those that are employed in jobs that don't give time off for voting or who don't have transportation to their voting location.

Voter intimidation is already illegal and there is no indication that it is a legitimate problem in the states that have postal voting.

Vote buying is also illegal and vote-by-mail makes it easy to document your ballot to get the promised kickback. This is logistically difficult for national elections but for local office it is much more likely to succeed.

Is there any evidence at all that vote buying is a problem in US elections?

No, but why worry about actual problems when you can make up excuses to disenfranchise people!

Switzerland, one of the most democratic places in the world has done this for a very lomg time.

While im theory its relevant a practical issue has never happened once as far as I can remember.

Going to hugly distributed areas to beat all voters up seems like a terrible attack vector.

So maybe in some places it could be a problem but I dont see it as much of a problem in a modern First World Democracy.

There are definitely smaller countries in Europe where parties have purchased the election by giving money door to door. I know it happened in Albania, along with threats of family members losing their jobs, etc. And they did lose their jobs once the election was over. There were threats to private sector employers if they didn't cooperate, etc.

You think people didn't worry about it before we implemented vote-by-mail? This is a solved problem, and it turns out that there isn't much of an attack vector here after all.