This is something those against Khan haven't necessarily understood: Khan has never been a hugely popular mayor, but his opposition has always been worse, which has helped his standing. Goldsmith smearing him in 2016 didn't help himself, Shaun Bailey said a lot of controversial stuff, and later transpired was strongly implicated in Partygate, and now Susan Hall is similarly poor, if not worse. Third party candidates maybe more competent, but then they don't have the advantage of a strong party banner and its prominence and source of campaign funding.