Actually, Gen 4's longevity had a lot to do with the DS. It turned out to be an extremely popular and long-lived handheld, which gave GF a lot of time to plan out Gen 4 and give equal attention to both Sinnoh and Johto/Kanto. The gen did get off to a rocky start with DP (which were a troubled production full of bugs and poor gameplay choices), but it ended very strong with Platinum and HGSS, which are both widely considered to be a return to form for the franchise after the Gen 3-early Gen 4 Dork Age.
The 3DS, on the other hand, hasn't been nearly as successful as the DS, and on top of that, Pokémon came onto the handheld a few years into its run. To be honest, I think the only way Kalos could've gotten a fair chance in this setup is if ORAS had never been made. XY were already doomed as soon as they got shoved into the background less than a year after their release to promote Hoenn, and GF spent a lot of time and resources remaking RS that could've otherwise gone towards polishing Kalos via Z or X2Y2.