They're employed by Hat Trick surely, so they can't be sacked by the BBC? And I can't for a second imagine Hat Trick sacking them. Ian and Paul are the show, they've been there since day one and the ratings are still good.
I've always thought if one of them gets tired of the show and decides to leave, then they'll probably step down together. Hopefully that's a long way in the future though, as I love them both on HIGNFY.