As you recognize, unless you actually go back post-election and ask every person whether they actually voted, you won't be able to detect successful in person voter fraud.
Do you really think that is more efficient than a voter ID law in effect?
We already covered this, what you're saying is completely untrue.
If people are impersonating other voters you will invariably end up with situations where an individual is recorded as having voted twice, or election workers will report someone coming in and trying to vote as someone who has already voted.
Even if that never happened you still wouldn't have to go ask every person who voted, you would just use statistical sampling techniques to catch them at a MUCH lower cost than any voter ID plan. Like, a small fraction of the cost.
Seriously, the idea that you can't catch in-person voter fraud is ridiculous. Now that we know catching it is very very possible, shouldn't step 1 be to see if it's happening before passing laws like this? Wouldn't that be the rational thing to do?