Iranian centrifuges had no network connectivity: https://en.wikipedia.org/wiki/Stuxnet
this is one of the biggest straw men i've ever seen on this site.
you realize that building computers out of various plastic and rare earth materials sourced from all over the world isn't free, correct?
What happens when a bug occured and machines need to be updated? Do you bring all machines in for maintenance or does a local IT shop do it? Maybe the manufacturer send a person but can the person be trusted to update multiple machines all over the country?
They (generally) aren't.
(Almost) every demo of voting machine hacking involves physical access to the machine, tools, and keys/lockpicks.
All electronic voting machines should output a paper human readable 'scantron' type ballot that is safely stored for verifying the internal digital tally.
And then that ballot should be what is actually tabulated and the internal digital tally ignored, or not even kept. The machine should be a device to facilitate creation of accurate, easily tabulated paper ballots, period.
We should just have paper scantron style ballots that people fill out. Then have scantron counting machines to tally them up after all voting nationwide has closed.
All the other issues people are trying to do to "fix" some aspect of voting are fundamentally flawed and those who are advocating for them are either desiring the ability to violate election integrity or don't understand the requirements of voting. Or trying to get money because someone in power sits in one of the two prior camps.
Vote counting machines are just as problematic as voting machines. We should just get rid of all of the machines and both vote and count by hand.
This is a very important issue, certainly. There is no excuse for voting machines not to be air-gapped. But that's not the only very important issue with voting machines.
If we're really concerned about this - get rid of the network connectivity.
Obviously this is over simplified, but just make a simple locally ran app or web application that refreshes once a vote has been cast. Store the vote in a simple SQLite database and at the end of the day, have a voting official go to the machine, and unlock the case, press a button on the back, which prints out or shows vote counts on the back on a small screen or something. They then can report that to whatever commission, committee, or otherwise is monitoring the election.
It's all locally stored, maintained, and under lock and key.
Why does this seem so difficult? We don't need democracy hindered by "network connectivity", just make it more environmentally friendly so however millions of people times the size of the paper ballot isn't wasted needlessly.