I think this is the OPs point though. You are
trusting someone to make sure the ballots are in a locked closet. You aren't personally there to see it, and I personally have never been on scene to witness vote counting by a bipartisan coalition of judges. For all
I know, the ballots are taken in a back room, burned, and then the election officials throw darts at a board to determine who wins. There are no publicly available recordings of the count. I'm trusting quite a few people who say they were witness that the count was legitimate. But in my experience buying off the word of even dozens of people is fairly cheap for what you get manipulating an election.
And more generally, if you are already trusting said election commission to do the counting by being in a partisan staredown the same principle would apply to said commissions scrutiny over voting machines. If the parties are balanced enough in power to insure no one is stuffing the ballot behind anyone elses back they can probably mutually conclude if an electronic voting machine is safe to use for the both of them.
Its worth mentioning and considering that, even in those coalition election commissions, both parties are incentivized to discredit third parties any way they can. They share a duopoly of power that they are both in their own self interest to protect, and I do not hear often about third party oversight of election commissions. Why do you trust them on that front, then, when their incentives are entirely aligned against being truthful?