If you vote from home, what sort of receipt do you get? A digital receipt for every candidate can't be recorded unless it's mailed in (and if people are all mailing in anyway then it sounds like you're just overcomplicating things), and people keeping a record of their vote at home where they can show others will let people systemically validate blackmail - "show me you voted <candidate> or I'll break your kneecaps". Although to be fair, if you seize someone's mail-in ballot you could force them to write it out under supervision.

Other than possible cost savings, the only useful benefit of electronic voting is if it lets people vote from home/vote more conveniently. Requiring a paper backup nullifies that benefit, and making it optional damages the usefulness of having a paper backup in the first place and adds privacy problems.