The only theory I have that supports them including future generations lie in the Gen. I Pokémon that are capable of evolving in later generations.
These Pokémon tend to have very low CP ceilings in comparison to other Gen. I "final stages."
Examples include comparing Tentacruel and Seadra. Both Pokémon are technically in their final stages for Gen. I, but Seadra is capable of further evolving in Gen. II to Kingdra. As a result—at least for me, personally—Tentacruel caps out around CP1350 whereas Seadra caps out around CP1000.
Onix caps out around a measly CP500, and if the second stage achieves a 2.5x CP multiplier, much like it is for Tentacool evolving into Tentacruel, that would make a Steelix cap out in the CP1200-1300 range.
Chansey is even worse, with CP capping around 400. Hopefully there's a higher CP multiplier involved with Blissey.
A couple questionable ones are Lickitung and Porygon. Based on the ones I have, they both seem like they'll cap around CP1000, which is relatively high for a basic stage Pokémon. Second stage Pokémon like Ivysaur, Charmeleon, Wartortle cap out around the same, but they take 100 candies to evolve to their final forms. If Porygon can eventually reach Porygon-Z, I can't imagine what the cap would be. It could take the opposite treatment as the basic bugs like Beedrill/Butterfree, where they scale horrifically early on. Porygon could potentially scale horribly as well, but start off with a much higher CP cap.
All speculation, of course. It'll be interesting to see. I would like it if they could eventually introduce following generations one by one to hold players' attention. Right now they're playing the nostalgia card hard with older folks like me who grew up starting with RBY in the late '90s. This leaves out the market for younger people who may have started with Gen. IV, V, etc. Should they update the game with newer generation Pokémon, they would have gotten their share of attention from the older folks already, but play into the younger crowd and pull them in if they weren't already.