If the BBC were to commission a new open-door non-topical radio programme, I think it would be every bit as useful for discovering new writing talent as Newsjack is.
There are some crucial differences between writing topical and non-topical material. For instance, with non-topical material, the writer has to construct the characters and scenarios from scratch, whilst with topical material both are 'prefabricated' - so to speak.
Also, the vast majority of sketch programmes on both television and radio are not Newsjack-esque topical shows, but rather non-topical ones.