I hear this from time to time but a massive security hole is putting everything in one place on the internet.
The coordination and effort required to go door to door stealing ballots out of mailboxes or taking van loads of ballots from a post office, in a day and age where every block has dozens of cameras. There is also a step where you get a code, and you can log in and see how that ballot voted.
Think about the amount of money required to pay people to vote, say people will do it for $10, and everyone you ask that doesn't do it also keeps their mouth shut. You're talking about hundreds of thousands of dollars to influence state level elections and millions for country wide elections.
The beauty of mail, is that while there are attack vectors, they are expensive, easily noticeable, and have a very high barrier to entry.
On the other hand, say you have a perfect voting machine, I'm skeptical, but I'll give you that, it's certainly possible. Now, lets assume that they have to either be stored in a warehouse with armed guards, or reimaged every time they are used. There's a simple attack vector there of only one or two people you have to turn. This is the old crypto breaking with a pipe wrench vector, though cash money is probably just as easy. That is the same failure that paper has, but you have to go to hundreds of thousands of places to get that done. You can probably keep a couple people quite, but a few thousand?
Paper by mail has a very high tamper barrier and is available now. States could do it in November