You hit on something there and i just wanted to highlight something about it.
SOTN, did do a few big things and still manage to be a huge success. The game was much easier. The game was a metroid style rpg. The plot was changed in a very drastic fashion. And it did manage to bring in a new fan base that expected to see metroidvania style games. It was a huge success, even among the classic Castlevania fans. Now here is the rub.. And the reason i started a thread asking if people would give this game up if things would have remained the same.
Fans of the classic Castlevania games were ready for a break. It felt good to play an easy Castlevania game for a change. I admit that.. But SOTN was designed to be a one hit wonder. Sorry to tell everybody the truth here. That's why the system started to fail after this game. How many super easy, hack n slash, simple as dirt, rpg style games did they think they could make before people would just lose interest? The answer? Not many. It went downhill and never looked back.
The classic Castlevania style held up for many reasons. Difficulty is a major part of the formula. Difficulty gives a game replay value. These reasons are not present in SOTN, especially difficulty. Nor are they present in any other modern Castlevania title that was not just a remake of a classic game.
This is absolute rubbish imo. The metroidvanias varied in difficulty and out of them, one could say 3-4 of the games were easy. COTM was more difficult than a lot of the older CV games (with the exception of CV III), the next 2 iterations were fairly easy, Dawn was average, but harder than AOS and Julius mode was much harder. POR and OOE had an extremely hard difficulty for the average gamer with Level 1 max cap hard. Out of the 2, particularly with OOE, I debate that enemy and platform placement were most notable on the hardest difficulty. The only major difference between the 2 styles of gaming are the rpg elements and save points rather than levels. To say these games weren't designed to be difficult is just untrue. Iga even mentioned this mode prior to POR's release in EGM.
If you think simply that because it's not the default setting of the game that it wasn't designed with this intention, you are mistaken. It's not just CV, every gaming franchise which wants to be/remain big/commercial have incorporated an easy setting (even Mario games are now using Nobbit and white tanooki) The point being that if this ever was a downside or an issue, Konami did correct this by incorporating that difficulty in POR and OOE. Particularly when you don't play as the main protagonist and can't use potions etc, for most people, they wouldn't be able to complete those modes. In conclusion I don't see this as a valid argument, you can argue 3-4 of the Metroidvanias were easier than previous Classicvanias, but certainly not all of them. (That's like saying Muramasa for the Wii is easy, then you find out you have Shigurui mode.)
Metroidvania was contemporary CV, now that has become LOS, later it will become something else. It couldn't just be static and never try anything new, this alone would have killed the series.