Union culture is as antagonistic in France as in the US (for the same reasons that corporations are antagonistic) and it not only has multiple GOA4 metro systems running right now across the country[0], Paris is actually converting existing lines to full automation.
All of Germany (which has a much more cooperative union culture) has two GOA4 lines both in Nuremberg, meanwhile Copenhagen Metro (in highly unionised Denmark) was created fully automated back in 2002.
I see no evidence that unions have anything to do with it.
[0] quite literally: Lille, Toulouse, Lyon, Rennes, Paris
Unions in the US are annoyingly different: most (at least those that I'm even passingly familiar with) seem to have one major goal: keeping the status quo (with regular pay and benefits increases for its members, of course). They generally do not go for "hey, we're going to eliminate your jobs, but we'll compensate you in such a way that you'll continue to be gainfully employed elsewhere".
Also, as far as I can tell US public sector unions are as strong as those in any other rich country.
Many cities with autonomous trains indeed have drivers. But they don't do nothing, they sit there for emergencies and to set the go signal when leaving the station. In Austria for instance that's not because of unions but because the job of the driver is at this point considered important. For fully autonomous operation you need extra security features on the track which were not employed (fully sealed off track in stations, better emergency corridors, more reliable remote door controls etc.).
Where fully automated trains are in operation there are never any drivers.
FWIW Lyon's Line D is fully automated (GOA4) and doesn't use enclosed track. It may have changed since but it used to not even have turnstiles, you could walk up to the track without any pause.
The only incident I can remember is a drunk who literally fell on a train from one of the elevated passages over the track.
