It would be very difficult to introduce enough extra paper ballots to make a difference, because the chain of possession is audited in real time by a lot of people, many of whom don't know each other ahead of time and have no incentive to collude on a felony.

Perhaps you'll prove me wrong in a follow-up comment, but in my experience folks who think about paper vote fraud like what you describe, usually have little to no experience at actual polling places aside from popping in briefly to vote.

In contrast, you could have 20 people looking at an e-voting machine, and none of them could tell whether it's working correctly. It's a black box, literally in many cases.