Considering that, despite the huge difference in graphics between G/S and R/S, it didn't take more then 3 years, but it took 1-2 years more to release D/P, I'm betting FR/LG played a role in that delay. Plus, the DS isn't all that different when you consider that Pokemon games, at least in D/P, only used the touch screen for the menu screens, attack screen, and other minimal things like pokemon contests. It certainly wouldn't require a year or two of extra development.
My main point was that you said a new series or two could've came out if they chose to not remake R/B and G/S, which is clearly not the case, as the release dates were only delayed by a year at most, and the games come out 4 years after the previous ones, and that could be explained also by the higher level of complexity in the games as well as higher detail rather than that year difference being caused by the remakes.
The DS's hardware and how it handles things is VASTLY different from the gameboy series. The SDK shared not much if anything in common with the gameboy series, whereas the gameboy series SDK shared a lot more in common probably. This meant that they had to build everything all over again, whereas in earlier game releases they could borrow a lot more from the previous versions, so that would add quite a bit more time to the development cycle.
And sorry, I got the year wrong. D/P came out in 2006, so it is just one year more than the regular release cycle, not 1 or 2. And then as shown in B/W, it may just be one of the most advanced games for the DS from a graphical perspective.
What possible reason would Gamefreak have for remaking R/S? Besides being money-hungry? FR/LG came out after R/S, so they aren't that different. If R/S needs to be remade, FR/LG needs to be remade. And if FR/LG needs to be remade, D/P needs to be remade, and so on and so on.
Pokedex. R/S has pokemon that can't be obtained in other versions. Ideally the way they will "remake" them is by implementing the Hoenn region in a newer game, but somehow those pokemon specific to the Hoenn region need to be added.
You are arguing a slippery slope, by the way. All remakes thus far have happened on a need-to-in-order-to-complete-the-pokedex basis, that won't be changing (FR/LG exemplifies this with their ability to get the legendary dogs). No game will be remade until it is imossible to trade the pokemon in that game to the newest version of the game series. That is the way it has been, and that is the way it will continue to be. Since 3DS is completely backwards compatible, that means that the "worst case scenario" is that FR/LG and R/S will be remade, but since those can be obtained still if you have a DS Lite, it isn't a high priority probably. Not until the first 3DS game comes out do I expect to see one get remade, and that is another 3-4 years off. They may even do this in a superior way such as having Hoenn and Kanto available in the same game setting, as we know they share the same timeline.
tl;dr version: The re-releases have at worst pushed new games back a single year, not enough time to release a new title, much less two, as you postulated. Remakes are released only in order to complete the pokedex, so there is no slippery slope of titles being remade over and over again.