You do everything the same as today. Then you turn it over to QA who keep finding weird things that you never thought of. QA finds more than half your written bugs (of course I don't write a bug everytime a unit test fails when doing TDD, but sometimes I find a bug in code I wrote a few weeks ago and I write that up so I can focus on the story I'm doing today and not forget about the bug)

QA should not be replacing anything a developer does, it should be a supplement because you can't think of everything.

We also use QA because we are making multi-million dollar embedded machines. One QA can put the code of 10 different developers on the machine and verify it works as well in the real world as it does in software simulation.