WI is 85% paper, 15% electronic w/paper receipt
MI is 100% paper
PA is like...80% electronic w/no paper trail and they're from the 80's so no one knows how to hack them anymore
As has been pointed out numerous times: If someone, such as a hacker, were to tamper with an election, the individual voting machines themselves likely wouldn't be the target. There's other computers along the way which store and transmit the numbers, which would likely be easier targets. It doesn't matter if they're originally on paper themselves, as those are still counted by machines and stored there. Some may even still require humans to read numbers and manually enter them elsewhere.
That's not to imply anything happened, but if someone wanted to tamper or hack, the other machines would be a better target as they could just trigger a few votes across the board without potentially raising suspicion. A single machine or two with a large number of votes would raise suspicion.
The likelihood of this happening is very slim, but it's not zero, nor is it impossible. Having only paper ballots would at least enable hand recounts to spot any potential problems. People in generally terrible at computer security. I would not be surprised if some areas are still using windows 95 machines with dictionary passwords.
This may be a non-issue this year, but in the coming years this will happen at some point.