He has all the money he needs so does not need the patronage of the Tories. He can safely become an independent or move off into wherever his pals offer him a 'consultancy'. He can, therefore, afford to say something now as a marker for the Whips if the Tories are in the driving seat.
The party whips can't persuade, bribe, threaten, blackmail, etc., a handful of independently minded MPs who just refuse to toe the party line. Once the whips realise this they tend to get left alone. They usually have strong feelings on one or more issues or are just serial rebels. Perhaps Zac Goldsmith is one of them.
The question is: could he resign, cause a by-election and then stand for election as an anti-Heathrow independedent? No anti-Heathrow organisation has ever fielded a candidate in a local or general election, so there are no precedents.
Why is this? Clearly the anti-Heathrow organisations know something. Could it be that Heathrow expansion is not the potent issue that changes votes?
It certainly won't be in this general election.