Is it too soon? I actually don't think so. While they may be more recent titles, the truth of the matter is that once the series made the leap into 3D, it metaphorically aged the Sinnoh games much more. Add to that all the mechanical additions of gen 6 (Mega Evolution being arguably the biggest), and gen 4 is suddenly more outdated than it has ever been. And now that Game Freak, in fans' terms, loves to throw surprises at us, I wouldn't be surprised if we got gen 4 remakes as early as gen 7. I actually wouldn't have been surprised if gen 6 gave us DPP remakes in addition to ORAS, to be honest--it would have made the lack of content in both XY & ORAS a bit more understandable, at the very least.
If you ask me, not only is it too soon, but it's also just unnecessary.
I struggle to think of anything that could really be added to Sinnoh that Platinum didn't already add. The biggest issues with DP--the Dex, the glitches, the framerate, the NPC teams, the postgame, and the story--were all fixed in Platinum, so outside of Fairies and Megas, what more is there to add?
By that logic, ORAS was unnecessary, too. But fans wanted it, and Masuda has gone on to say that that was part of the reason they chose to move forward with the games. Fans of gen 4 exist, and people who want them remade exist. Couple that with bringing Sinnoh into the newest generation, and you have enough reason to remake the games.
Plus, there's always the chance that remakes would just be a rehashed DP and ignore everything Platinum added. HGSS may have embraced Crystal, but FRLG and ORAS completely ignored Yellow and Emerald, so past history would suggest Sinnoh remakes being another hackjob like FRLG instead of a quality, finished product like HGSS. Would anyone really want to see that happen? I know I wouldn't.
I agree with you there--Platinum is, to me, one of the best games of the entire series. So if gen 4 remakes were to happen but they treated Platinum the way ORAS treated Emerald, I would be utterly devastated.
But that's a rather silly reason to not want a game, don't you think? I mean,
any game has the possibility of turning out awful. But that shouldn't deter you from wanting a game at all. Instead of saying "I don't want a gen 4 remake because it could be handled poorly," say instead "I want a gen 4 remake that takes advantage of everything gen 4 has to offer--including Platinum."
After the NX comes out and we (presumably) lose compatibility with DS carts, I'd rather see DPP just get the RBY treatment and get a VC re-release on the NX (which I'm hoping will be powerful enough to have a DS emulator). If games as old and outdated as RBY can be made relevant, I don't see how the same can't be done with far newer games like DPP. The era of Pokémon remakes may have come to a close with ORAS.
In all honesty, gen 1 will always be relevant simply because it's gen 1--that's what started the whole franchise and TPC(i) thrives in its nostalgia factor. But games past that are only relevant when they're brought into the current generation--gen 3 is relevant right now because of ORAS, for instance. It would be tough to get people interested in gen 4 if all they did was port it to the Virtual Console (especially considering that it isn't one of the more popular generations). But by remaking it, people would get interested again since it would be fresh and up-to-date, and
that would succeed in making it relevant again.
And in regards to supposedly losing compatibility with DS cartridges, I don't think that would matter
too much, since gen 4 Pokémon can still make it into Bank, and TPC(i) has mentioned that they want to keep Bank as the relevant "transfer method" for as long as possible. The people who start with a system that is incompatible with DS cartridges probably didn't have any gen 4 games to transfer Pokémon from to begin with.
Not to mention, in addition to all of the above, that remakes are generally easier to make because of how all the source material is already created, and now it just has to be "re-crafted," in a sense. Remakes give the fans what they want--a new foray into a region/game that they love--while also allowing the developers to make a game that won't be as time-consuming to make as an all-new game that needs all-new characters, assets, levels, etc. to be designed from scratch. Like, they could make a remake while simultaneously going through the pre-production period of their next new game. So it's actually beneficial to the developers in a sense, too.