To be honest? He had PLENTY of time to make the Demon Castle wars.
Also, you are assuming he ever intended to make it at all. It's nothing new for games to have big background events pertinent to their story which are never explored as games.
Mega Man has the Elf Wars, the massive cataclysmic war between the X and Zero series, which has never been made a game, and never will be, because it's just that. background. It exists to set up the plot, for world building. IGA decided, "Hey, let's make a game about Dracula's reincarnation that plays like Symphony"
When ironing out the story for that, particularly when the Character of the current Belmont comes up, they concoct a backstory that ties him with the protagonist and the setting and timeframe of the game. That event being this massive final battle between Belmonts and Dracula.
IGA had plenty of time to make it, instead he made DoS, PoR, OoE, and 2 PS2 games, along with judgement. That's 3 sequels,(to games that did not need them) a prequel, an original timeline fit story, and a fighting game that's iffy on the plot and uses time travel as an excuse to toss a bunch of CV characters together. Then there's HoD, which i dont know what to call other than an odd experiment. That's a total of 7 games. 7! Instead of making Dawn of Sorrow, he could have made the Demon Castle War. Instead of making PoR or CoD, both sequels which shit on the games that came before them, he could have made the DCW.
He had his chance quite frankly, and he blew it. That is, again, if he ever intended to make a DCW game at all.
You mention that CV3 remake too, well, he had his chance, again. Instead of making those games, he could have made a CV3 remake like Dracula X Chronicles.
so to finalize,
so do you feel that IGA was robbed?
Nope. I think he had his chance.
why has MercurySteam been given the opportunity to finish their saga, but not IGA?
because their saga is only 3 games long, while IGA's was never ending. It's hardly comparable. LoS is also outsourced. MS is doing the developing, with Konami just supervising and publishing. So money works a bit differently.
Also, IGA was building off of the old timeline and trying to cram as much as he could into it until it burst, and not all of it for the better. Bloodlines and CV3 did NOT need sequels. neither did Aria. he wasted time and money on needless sequels that were very poor in terms of original content, full of rehashed graphics and gimmick driven gameplay. As a result, Castlevania sold less and less. After all, he didn't try to appeal to younger gamers on a whim with Dawn and Portrait. it was because sales were staggering, so he tried to widen the audience by appealing to little kids. Itdidn't work and he tried to go back with OoE, but too little too late.
Meanwhile, LoS is supposedly the best selling CV game of all time. So obviously MS gets their chains loosened a little to flex their muscles with the sequels. Which from the start mind you, they said would be a 3 parter only.
The choice of platform also counts immensely. IGA was working on handhelds And while the GBA was the hot thing to have back in it's day, (it was very much the ONLY thing to have, since Nintendo still had the stranglehold on the handheld market) the DS, with competition from the PSP, became branded as a kiddie system, that had kid games. people who before had only 1 choice, flocked to the Sony handheld that had superior hardware and was marketed as a cool system, while Nintendo was doing that thing with the apple style designs and casual market appeal.
TL;DR on that- the DS was, especially late into it's life, branded a system for little kids, so not as many people who would normally have bought CV games, bought them, because of the system. Not that the PSP actually fared much better when Konami tried it. But the PSp had a very weird sell in Japan.
Console games meanwhile, are whats considered top shit, and IGA's foray into 3Dvania was received with mixed emotions. lament was alright, but didn't look that great, and was sort of weird. CoD compounded LoI's problems with more problems, including story and a character so blatantly trying to imitate Alucard that he makes Juste look tame by comparison.
Then Konami made the gamble on Judgement instead of the currently in early production Western pitch of a CV1 remake, which was re-branded an original IP as a result. E3 was all about Judgement for Konami that time, with Lords of Shadow the side next gen game. It was a Symphony moment, basically. They bet on Judgement, with LoS as the side game, and judgement was a critical flop, while Lords of Shadow went on to become a best seller when it released.
From there, business sense. IGA's games got worse and worse results, while the outsourced studio got great results. So IGA was given a last chance, which turned out to be CVHD as an experiment. It predictably got mixed reception for different reasons, and was overshadowed by Lords of Shadow.
The promotion on LoS was also really well done. I don't recall CV having that level of promotion before.
So there, that's my 2 cents. Well, more like 10 bucks.