Plus such coercion isn't impossible now, like you said it can be done with a camera. Presumably if you're being coerced by threat the "illegal" doesn't really help you at all.

It seems obscene to me to pass up such an obvious improvement like a verifiable record over such a theoretical concern.

1) Coercion is literally the biggest concern within elections. You put a big guy with a pair of brass knuckles outside of the polling location and he beats up everyone who comes out without a picture of his ballot with the right vote in place.

Theoretical? Lol. This WAS and IS common. This is why we have international election observers.

2) There's no integrity loss with paper if things are done right. Every ballot given out at a voting booth is recorded - you can't stuff the box as a voter. The only way you can do so is if election officials from all parties collude to fudge the books together, which they would need to do in view of the public as they man the voting desks, or under the supervision of the locale manager during the end-of-night count.

Paper.