Well here's the thing, the reason why Castlevania is commonly a action-plateformer was because the game originally started out as a side-scroller which was the standard model during the 1980s and early 1990s due to the extremely obvious limitations until 3D world spanning games came until much later, all that time they were trying to imitate the platform genre foundation it was originally built on with some or little successes.
Call me a progressive or 'sacrilege' at this point but Castlevania is whatever you want it to be which in this case, engine technology has moved on to the point that plateformers and side scrollers are kinda 'irrelivant' and pretty much optional than the standard requirement as it was during the 1980s/early 1990s due to limitations which in this case, they could might as well abandon the plateforming elements in Castlevania and just make it a fullblown Action-Horror RPG game of Dark Souls and Bloodborne which could be a nice experiment.
I don't think it entirely matters what the limitations were or whatnot back in the 1980's. The fact of the matter stands is that that's what Castlevania was, and a large majority of Castlevania IS.
Take a look at Mario, for example. Mario has always been a fast-paced, precision platformer (albeit a bit too slide-y). It began this way with Super Mario Bros., and even after limitations were lifted on newer generation consoles, the Mario series stayed this way (with the exception of games like Super Mario 3D World, which are a bit more slow-paced and down-to-earth). Zelda is much the same way. And even other titles like Metal Gear pretty much remained at their roots for most of their franchise (though with The Phantom Pain it's a bit debatable).
Castlevania being "whatever you want it to be" due to limitations being lifted is a bit of a weak point to make as far as where the series could or should go. For example, what if I wanted Castlevania to be a music rhythm game? Like the Souls-styled game, it'd be a nice experiment (well, for this one, "nice" is debatable), but it wouldn't really be
Castlevania, would it?
I also wouldn't consider platformers "irrelevant". Mind you, Mario still sells like hotcakes. And even lesser known ones like Yooka-Laylee are getting a lot of hype. Castlevania doesn't need to remain a 2D-platformer. It can be done in 3D, despite the fact that it
hasn't been done yet.
Mind you, I'd be all for a
spin-off in the vein of Souls (or a rhythm game), but as for the main series, I think it should remain being action-platformers.