It was never realistic for him to stay when Brentford wanted him on a 6-year deal. Bowen probably would've gone earlier too if he had the chance, and Greaves I'm sure would go if a PL club came in with a serious offer too. Players just very rarely reject these opportunities. It's easy for us to argue they should from a distance but when they weigh it up, training with top players and coaches at top facilities every day, the chance to play on the worldwide stage against the best players, more money and security and so on, it's generally an easy decision. Throw in Brentford's record of improving players in the long term and it's even more obvious. No one outside of Hull would've expected him to turn it down.
Oh no doubt, but KLP was making noises well before Brentford came in with a solid offer.