You can't stop a series' evolution though. Nothing can stay the same forever. Even 2.5D mario has evolved significantly since the days of SMB.
If you're referring specifically to the artwork then it's a different story. It probably would have worked for games which featured Belmonts as a main character, aside from Juste.
Personally I can't fault how the art evolved and I do believe that both LOI, AOS, COD and OOE have some of the best video game art I've seen.
The barbarian art was cool for its time but not every Belmont needs a bowl haircut like Heman and Conan's physique. Also I don't know about anyone else but CVII's cover art seems much more feminine to me than CVI or III.
Every ongoing series needs to evolve with the times to stay relevant, and around the time SOTN was coming out, the hyper masculine male heroes were out, and the more feminine and vulnerable male heroes became what consumers wanted out of their stories. I will always prefer the original style, but you have to admit, people would have a hard time taking an exagerated style like that seriously nowadays. You never really see it unless its meant to hearken back to those times or if it's not supposed to be taken seriously(like some modern movies having old school inspired posters on their Facebook advertising campaigns). It's a shame because most art from that era is simply awe inspiring, such as Frank's up above.
Plus, you're absolutely right that Kojima has provided the series with some of it's best artwork. My personal favorite of hers is probably SOTN or Aria. It's provided the series with even more fans, and even people who don't play CV love it (my girlfriend says her favorite CV character is Leon because "he looks so cool", she's also a fan of Alucard)