Are you seriously arguing that a userbase difference of 130m systems doesn't mean anything? And you do understand that RE4 was a late port right? Gamecube had exclusivity for nearly a year. Late ports almost always sell worse than the system they were originally on. The last two REs on Playstation sold 8.5 million units combined. The next two on Gamecube sold 2.5 million. That's a pretty massive drop. If you want to keep arguing that the Gamecube didn't hurt the franchise sales in any way then feel free but it makes absolutely no sense.
While being on the PS2 would have been better for sales, I really don't think it's as much as you think. RE3 was already on the decline from RE2 and that was a mainline entry. Of course a remake and a prequel weren't going to light the charts in the same way.