Considering horizontal gene transfer, I would say there most certainly is a route for stated existential threat. Bacteria trade plasmids back and fourth all the time, including those that code for antibiotic resistance. So while you are correct that the most likely outcome is a return to the pre-antibiotic era (still really bad), you underestimate the dangers in the resistance in that all sorts bacteria can become resistent, both against the medications themselves as well as against the body's own immune system.

Also, notice I intentionally said an existential threat to society, not humanity. It's fair to say that no longer being able to perform surgical procedures safely would be very destabilizing to modern society.