What's the role of the software engineer besides setting this up?

Your example makes me think it will merely moves QA into essentially providing countless cases and then updating them over time to improve the AIs data.

And is it really gonna be cheaper than human support?

What's gonna happen when we will find out (see the impossibility to reach a human when interacting with many companies alredy) this is gonna bring (maybe, eventually) costs down, and revenue too because pissed off customers will move elsewhere.