If Starmer survives this then Sarwar absolutely has to resign himself. There's no way he can go into a Scottish Election campaigning for the electorate to vote for a Labour Party that he, himself, has openly declared he has no confidence in the ultimate leader of. If he tries to brazen it out it would just reiterate that SLAB leader is a position of absolutely no consequence whatsoever.
Also telling that not once did he mention Mandelson, the actual thing that precipitated this scandal.