Most of the fan base is around 15-20+years old Y'no those who grew up with pokemon and those who did grow up around R/S/E most likely have younger siblings who would know about the 3rd gen due to 15-20 year old fans.
That's mindblowingly wrong, whether we like it or not. A large proportion of the active fandom is in the age range you suggest, sure, but the market for the actual games is incredibly broad - and a good proportion of the audience they market to is, really, the younger end - 8-13. Just watch the anime some time or pay attention to the marketing for Pokémon as a franchise. Indeed, a great deal of the people who sign up to our very site are below your target age range. Ask any of the kids who signed up for the Trainer Card Maker, we have a lot of 11, 12, 13-year-olds who use this site. Who would have been too young to experience Hoenn the first time around when it first came out. They might have played it, sure, but the vast majority won't have.
Which is actually why they're doing the remakes - they're producing a game that's nostalgic for anyone who played the Hoenn games, but it actually ends up being a brand new experience for anyone who hasn't. Either way that translates into lots of money for Nintendo, and it really means both audiences would be interested in the game. I've never played RSE properly, so I want to experience the region for the first time. Many people I know have, and want to see it updated to modern graphics and re-experience a game that they first played ten years ago.
And really, for a personal note: Sure, I want Hoenn remakes. But side games are an important part of the franchise, and some of them are quite good. Some of them are truly awful, but that's life. Slating this detective game now is pointless, you have no idea how it looks or what it plays like - this is an early proof of concept, without a visual style, solid gameplay, or even a premise we know about. You're seeing visuals that you don't like and jumping to conclusions based on guesses.
And I can make the same point I did earlier: Making this game and making the Hoenn remakes are not mutually exclusive. I can tell you for a fact that they're working on several games right now - they were working on XY (and Harmoknight!) when B2W2 were released, and I can lay good money on them working on both the Hoenn remakes and the next Kalos game right now, just with a bigger dev team for one. We will get all of them in good time, and they're not sapping precious resources by working on other things.