HAHAHAHAHAHAHA
Oh wait you're serious...
Variation IS a good thing. and the IGAvanias definitely gave us a few real good characters, Belmont or otherwise. (some better than others)
But Belmonts NOT being necessary? They are the staple characters of the series. Them and Dracula. or at least Belmont stand-ins, like the Morrises, or Nathan Graves from CotM. You can make a side game or two where they play little to no part, but saying they are not necessary to make a Castlevania game is like saying Samus is not necessary to make a Metroid game. (Or to an extent, Metroids themselves, which the plot of the entire franchise revolves around- Even other M and Fusion, which take place AFTER Metroids have been made extinct.)
To have the argument you're having, you need to clearly distinguish what Castlevania is. Unfortunately, Castlevania is a bit harder to define than other franchises since it has evolved a
lot during its lifetime.
I don't know about the rest of you, but this is what Castlevania is to me:
1. Gothic atmosphere
2. Melee combat system
3. Platforming elements
4. Involves Dracula and his Castle
All four of those things have been core to the franchise for ages. Yes there are a few exceptions, but they are few and far between.There are other things I could add (music, RPG elements, .etc) but for simplicity's sake lets stick with these four. LOS did not have gothic atmosphere, it barely had platforming, and I don't even think if involved Dracula that much. I really feel like LOS was a completely different game that had the Castlevania brand stomped on it to make it sell. It's not Castlevania to me.
SOTN, on the other hand, had all of these things and more. There's a reason why it's a series staple. It added new elements to the series while maintaining everything that gave the franchise its character. A true sequel.
MOF at least maintains all of these elements, even if it doesn't do it extraordinarily well. I don't see any reason why it isn't a "true" Castlevania game. I also don't see any reason why Castlevania has to involve the Belmonts. It's not like there's one iconic character that represents the franchise, like Samus does for Metroid. A Metroid game without Samus would be daft, but Castlevania introduces new characters all the time.