Cech and David Luiz moved after their primes. They’d already had success at Chelsea and weren’t good after they left. Arsenal thought they could do better than Giroud and were happy when he left. But he obviously was an important player for us and aged well.

Whereas Ashley Cole came from Arsenal’s academy, I’m pretty sure supported them as a kid and was one of the best players in his his position in the world. And was slap bang in the middle of his prime. From Arsenal fan’s perspective he left for money although it was probably more the principle and feeling valued by the club.

Chelsea fans feel exactly the same way about Mason Mount. Him going to a rival isn’t what made fans angry. It’s the fact he’d been at the club since he was a kid, had already had success with the club and was meant to be a key player for them moving forwards. And the fans feel he chose money over loyalty (again probably more to do with feeling valued).

I don’t think Chelsea or Arsenal fans would care unless a player is basically leaving for money or because they think they can win trophies at the other club. So Van Persie going to United is probably more insulting to Arsenal fans than any of the Chelsea ones you listed. Or Fabregas going to Chelsea after Barca. He was already an Arsenal legend and had left in a bad way. Then returned to London to play for Chelsea (although Wenger didn’t want him).

The rivalry means nothing if the player isn’t good enough. Havertz isn’t even in his prime yet but he wasn’t living up to Chelsea fan’s expectations and you could tell he needed a change to reignite his career.

And yeah they hate Spurs a lot more than each other. But the lack of transfers between them and Spurs comes from the Spurs end and Daniel Levy.