I'm willing to bet they are just ports/remasters. I don't think Konami has enough manpower to make proper sequels for their flagship titles other than PES. Just look at MGS Survive or that Contra game. Porting classics is cheap, safe, and can be tasked to 3rd parties who will have the manpower needed.
Also, if they ever decide to remake the Silent Hill games, would they have to remake them from scratch? One of the reasons I heard the PS3 remaster was so bad was that they had lost most of the code.