Sorry to hear your troubles with a well looked after C207 - really surprising.
From my reading around I have seen quite a few posts in the last year or so that MB are often replacing these subframes FOC as a gesture of goodwill..... looks like the issue need to have visible rust and holes showing it is about to fail. Ofcourse this is no comfort when the subframe can look perfect from the outside and you can't see what is inside, unless you take a borescope with a built in light to investigate - this is beyond most home DIY'ers and could you trust a MB dealer to do this for you unless you are there to watch them?
@Fastcar155 what year did your subframe fail and did it collapse on you or fail an MOT to get written off?