To be fair, when one of your drivers ends up being first, and the other ends up being 8th, you kinda know that you have to fire him.
Perez was a great helper for a few seasons there, but the one before last season he started to be bad, not at the start but by the end he was terrible and when we look at last season he was more than terrible so we need to realize that Perez was done with the F1 world and he wouldn't even find himself a place.
That's usually what second driver means, you are as good as how much you help your teammate, and when you don't anymore, you are just out of f1 completely.
Lawsom may or may not be better, but that's not the point, even if he sucks, that doesn't make Perez sacking any less true decision.