There are arguments both ways as to whether Bellerin was fouled, however one thing is

[Sir C]

I'm not so sure - Alonso looked only at the ball and his arms weren't in an unusual position for someone that was jumping for the ball.

If we had a goal disallowed for that I'd be pretty irritated.

Alonso could have been looking at your mum's balls for all the difference it makes. He knocked an opposing player unconscious with his forearm. If that isn't a definition of dangerous play, I'm not sure what it.