If this does include only the 151 from the start, I think that along with a couple of other factors would explain it:
- As you said, this is clearly targeting older fans. There's one younger person in the video, and he's with his parents; everybody else ranges from late teens to like early thirties.
- Many of these might be lapsed fans. Throwing ~600 new Pokemon at them at the same time would be incredibly overwhelming, and would make fan favorites (most of which hailing from the first generation) much harder to find because unlike the games, their favorites might not be just a patch of grass away.
- This game will be much, much slower to "progress" through than the mainline games are. In the games, towns are several houses and a gym, while routes are essentially small strips of land and grass with trainers on them filled with a variety of Pokemon. With filling the Pokedex, what might have taken you a few weeks would take much longer to do in Go due to the spacious of the real world (and with that, Pokemon placement) and time constraints that would keep you from exploring outside at all hours of the day. With that, collecting the Gen 1 'mons will probably be enough for the first several months in a way that it wouldn't be in a regular game.
- There isn't going to be a "Pokemon Go: Gold and Silver" or "Pokemon Go 2" because there's hardly any graphics to update nor new regions to explore. This game's updates will rely solely on additions to the roster, which come from one, mega evolutions that you can't find in the wild and two, new generations which come once every three to four years. This game can't see big updates only once every generation; it needs a continuous stream of updates to keep interest up, and if the updates are mostly roster-related, you don't play your entire hand at the very beginning.
So yeah, I think this could be the originals at the start plus maybe Lucario and Zygarde for popularity and promotional reasons. After some months, they could hit us with an update adding another gen and more potential event Pokemon.