I imagine his judgement is that he will win more seats elsewhere. He may be right or wrong, but it is not an idiotic position in itself.

More serious, in that respect, are the 28 Labour MPs demanding Corbyn's reinstatement. If a substantial number of these resign the whip - or a similar number resign if Starmer does reinstate Corbyn - then you are not talking of gaining one seat to replace Corbyn's, but perhaps ten that need to be replaced. That's a much higher barrier to surmount.