You make it sound like IGA wrote all these games by himself
Hardly. I never said such a thing as I know better. Ya might wanna read my posts again if that was the conclusion you got. Legends, CotM and the two CV64 titles were handled by other directors. IGA didn't like those titles because a) he didn't make them himself and b) they don't fit into his story chronology which he was working on at the time. If anything he saw them as a hindrance to his work and thus retconned them. This doesn't include the mobile CV game which he helped oversee of course. That was not intended to be part of his timeline either even though he was involved with it.
I think it's pretty clear people at Konami never had the intention to make Belmonts the only ones who can kill Dracula also I'm pretty sure IGA based his work on the already existing lore from his team at Konami and it's pretty faithful work.
It's mentioned in the Bloodlines instruction manual that Eric Lecarde is in pursuit of Countess Bartly due to the fact that she turned his beloved into a vampire that he was force to slay. Eric's goal wasn't Dracula. I would say that he assisted John Morris in helping to fight Dracula but was not the one to deal the final blow. Story-wise mind you, story-wise. Gameplay is different.
I say it's for the best that there are other powerful non Belmonts out there because it wouldn't make sense that they are the only ones that can fight creatures of the night.
There are other creatures of darkness out there that other heroes could tackle. Absolutely. It'd be a somewhat lacking world if otherwise. But Dracula is the one being who no-one else but a Belmont could ever hope to best. That was pretty much established since Castlevania's inception. Otherwise past games that were made before IGA came along would have featured non-Belmont heroes.
Sometimes the Belmonts may produce a weak heir, does that mean the End of the World?
No Belmont has ever produced a weak heir. In fact I've never heard of this before till you brought it up just now. The magical properties of the Belmont family blood are not dictated by genetics. It's magic after all; God's blessing (I believe it is anyways as it makes sense). This magic ensures that if one member of the family is not available to counter Dracula, then another member will step up to plate and take him down. All Belmont descendants have this magic flowing through their veins regardless of what family name they carry. As long as they are tied to House Belmont by blood decent they will have the ability to counter Dracula. But again, IGA wanted to tell his own stories and brought in several non-Belmont characters to be the heroes whom defeated Dracula. I just don't agree with that part is all.
I agree on the Japanese complaint but the European setting has been capped because I don't think Dracula would be cool in 2000 onward so Aria of Sorrow was a good idea by IGA
I don't believe there would be any issues for Dracula not to be cool in the 2000s and beyond. Dracula is one of the most popular villains in Castlevania (if not THE most popular) and I seriously doubt that that's ever going to change. I actually have no real issues with the Soma Cruz character. I fact, for many years, I had wondered what it would be like to play as the infamous Count. But there is much about AoS that is Japanese-themed, and as a European-themed game it does skew things a bit in terms of the game's identity. This is Castlevania here, not Getsu Fūma Den.