You make a good point, but still, I'm not fully convinced that Mega Evolutions are going to completely take the place of giving new evolutions to old Pokémon. Even if Sylveon does turn out to be the only new evolution we get this generation, the fact remains that we don't know that for sure yet. Remember Generation IV? Remember how we got, what, Weavile, Roserade, and Electivire before the games were released? And now remember just how many other new evolutions there turned out to be? Honchkrow, Ambipom, Mismagius, Gliscor, Leafeon, Glaceon, Magmortar, Rhyperior, Froslass, Gallade, Yanmega, and Mamoswine, and that's just off the top of my head.
Basically, I'm saying that while you could very well be right, we can't say either way with certainty just yet. I do believe that the existence of Sylveon gives hope for other Pokémon gaining new evolutions, but right now that's only my opinion, and I could just as easily be wrong about that.