What was so unbelievable about it? I think it was exceptionally well done. You can believe it was supernatural or you can believe it was a hallucination and he simply died from heart failure.
It would have been justified if he was having hallucinations but when the spirit began to affect Ryuu then, personally, I felt that that was ludicrous. The story is based on a certain theme and environment. This abrupt instance of spirit possession was unnecessary in the emphasis of such a of death.
I think they have a point actually, I have nothing against supernatural but it seemed out of place in this Manga tbh!
Personally I'd like to think it was a hallucination but he actually could affect ryuu physically...so that clears it up!
*spoiler*
This is how he dies?! Nowhere does it say that this manga is supernatural. Sensei should have at least made the scene believable.