While IGA states that only Dracula's evil intent is amplified and brought back from the Abyss, I think he didn't mean it in such a literal way as the boss, Chaos, assuming control of Dracula and overriding his will. I believe IGA meant that the evils of humanity bring Dracula back and make him evil again.
Those two things are literally one and the same. Boss Chaos is only an extension of Full Chaos which applies most immediately to Dracula/Soma. It's tied to him like Edward Elric's Gate is tied to his alchemy and his alone. If Ed can destroy his own personal Gate, why can't Soma destroy his own personal Chaos?
Another thing comes to mind. Souma beat Chaos, but as he is evidently not as powerful as Dracula was, this again raises the question on how an entity like Chaos could control and override Dracula's own will, if he was truly unwilling.
Soma had the humanity that Dracula lacked. Humanity and goodness is what has kicked Chaos' ass through Dracula all these centuries--why is Soma the exception?
With his massive power, he could beat Chaos, even if bereft of his absorbed souls. It is evident that Chaos cannot assume control of another without a fight, as it did need to fight and defeat Souma for him to become the Demon King.
Where is it listed Dracula could actually beat Chaos? He can beat the evil god that held reign before him, but a singular all-powerful entity and an amalgamated pseudo-all-powerful entity that will continue to exist and grow as long as humans have the ability to be assholes are not even
close to being the same thing.
Imagine Menace, which was an amalgamation of all the evil souls and shit in Dmitrii, right? But this time it grows
perpetually because it's fed by humans, and can more or less recover itself from injury as quickly as it's injured because
its power is fed by humans perpetually. Soma already expresses that regular Menace is too dangerous and powerful to get out of the Abyss--imagine a Menace playing on God Mode with
all the cheats turned on. That's more or less Chaos. You can't kill what exists in a perpetual state, you can only kill parts of it, which will eventually come back
if the proper fuel source continues to burn.
Boss Chaos is Dracula's "part," and it continued to come back because Dracula kept coming back. But if Dracula can't come back anymore (in the form of Soma refusing the resurrection cycle), then killing that part of Chaos will kill that part for good, because its fuel source has run dry.
And Chaos assumed D I R E C T C O N T R O L all the time without a "fight." Not-evil Dracula might not have wanted to wake up this or that particular time, but that's hardly a "fight." You never saw a resurrected Dracula struggling with an internal battle of the mind and will, did you?
Dracula gets "overtaken" by Chaos because that's the deal he unwittingly signed himself on to. Whether or not he's actually "strong enough" to best it in combat isn't even a factor. You can't punch a malignant tumor out of your body no matter
how physically adept you are, and that's
exactly what Chaos is to his soul.
You wanna live forever and have all this dark power? Great, here's a
really fucking big monkey for your back. No, you can't pull it off by yourself. Don't even bother trying. Not even that Sinbad trick with the wine will help you here. Hey, no complaints. This is what you signed up for. It's your own fault for not reading the terms and conditions closely enough. Deal with it.
They create their own doom by having evil desires, which empower Dracula and make him reborn every 100 years or so.
Hundred years thing is
absolutely 100% false. It's mere happenstance that people rez him in roughly that timeframe. He's not bound to any resurrection schedule beyond how quickly the dark priests and cultists can bring his ass back.
The Chaos in-game is not an all-powerful demon, but simply a "battle in the center of the mind" taken literally for the players to enjoy a final boss, I believe.
Nobody ever said Boss Chaos or even Full Chaos was all-powerful. Boss Chaos is a physical manifestation of a singular facet of the whole thing, and thus can be slain accordingly under the right conditions (which Soma meets). It's not a mind-battle by
any stretch of the imagination. If it were a mind-fight, there would be
absolutely no need for a Chaos Realm, a portal to the Chaos Realm, an uber-powerful Chaos Realm Portal seal that only Dracula (and apparently Julius but it's Julius and fuck you he does what he wants) can pass through,
and a magical conduit to Chaos sealed again in an empty solitary chamber behind a special boss door addled with presumably stupid powerful magic (what else was all that lightning on it?).
By all means believe this, it certainly makes for a cool perspective on the final boss, just know that there's nothing even remotely supporting it as plausible.