No one is going to come to the AL and pich better, because you face 9 real hitters. Their ERA's will most certainly go up and they will not be able to pitch deep into games. We got a 37 year old Schilling from the NL. His ERA went up in the AL, but he was still a top pitcher that first year and he was a CY Young candidate. Everything is relative. Yes their ERA's go up, but a good pitcher in the NL will be a good pitcher in the AL. Their numbers will be worse, but overall AL pitchers numbers are worse.
Also, the list of hitters who switched to the NL and did better was very small and unimpressive. It proved nothing.