Is "then to do what" common/idiomatic?

Question

Example:

Speaker A: “I’m not here to catch you.”

Speaker B: “Then to do what?”

I thought it was common phrase. But Google tells me the opposite.

If it's not common, what's a better alternative?

Answer 1

What I would expect here would be:

Speaker A: “I’m not here to catch you.”
Speaker B: “Then what are you here for?”