Gen 2 was designed to compliment gen 1's group. And adding things to catch unique to post game was part of the reward for beating the Elite 4.
This isn't an argument for the merits of how Game Freak spaced out the new Pokémon in Gen 2, it's just a restatement of how they did it.
There were other options for how they could've done it.
For example, they could've had the Gen 2 Pokémon throughout the main game and then have the Gen 1 Pokémon show up in the postgame—and then
that could've just as easily been a "reward" for players who didn't have access to a Gen 1 game and another system to transfer Pokémon up to Gen 2 (and for any new players who hadn't played Gen 1). To new players, those "old" Pokémon would've been plenty new to them (and there are always lots of new players with each new generation—even before this had been established as a pattern, this is still exactly what Game Freak was hoping for with all of their advertising at the time). It also would've fit with the worldbuilding up to that point, instead of forcing people to explain why the new Pokémon mostly show up in the old region where they hadn't been (in the games' lore, or in our real life experience of the Gen 1 games) just a few years before.
If Gen 2 Pokémon had been all or most of what was available in Johto, then they would've provided novelty (in a new game, after all).
If Gen 2 Pokémon had been available alongside Gen 1 Pokémon, then they would've provided variety for both teambuilding and encounters.
If Gen 2 Pokémon had been more present in Johto, then they would've provided stronger worldbuilding to this new, separate region.
And I'm sure that there are even more arguments than what I'm thinking of now.
I don't want to argue entirely from novelty, since I think that people really overvalue just having novelty for novelty's sake, and novelty isn't everything—but novelty isn't worthless, either. I think that a lot of GSC players would've really liked to have caught and used a Houndour on their team instead of a Growlithe (again). Or to have at least had the
option.
But Growlithe is the one found in the early-game routes in Johto, while Houndour is stuck on a single postgame route in Kanto, at very low encounter rates, and only at night. They weren't just restricting a lot of their content in the postgame, it really feels like they were
hiding a lot of it, too. It's bizarre and frustrating.
Johto is full of gen 1 pokemon because they are the pokemon.
I understand this feeling, like the Kanto Pokémon were the common, default wildlife of the Pokémon world. But it seems like a post-hoc justification.
In Gen 1, those Pokémon were all that we had, so there was nothing to compare it to.
In Gen 2, Game Freak didn't
have to use those Pokémon again, but because they did, we're left to try to come up with explanations for it.
Also, all of Gold & Silver's
advertising emphasized the
new Pokémon that there were for players to discover.
It wasn't a franchise where they shit out 100 new marketable creatures every year to see more toys.
The ads sure made it seem like that's exactly what they were doing. They kept repeating some version of "new games, new Pokémon" every time, making the two feel synonymous and building the expectation.
Gold & Silver ads showed pretty much
exclusively Gen 2 Pokémon (aside from the inescapable Pikachu). They made it seem like this new region would have its own ecosystem of new Pokémon. I was pretty young at the time, but I remember that this was the impression that they were giving us.
For Game Freak to have actually hidden away so many of their new Pokémon in the postgame feels like a weird bait-and-switch.
Maybe it was meant to try to make the games more mysterious, to give kids hard-to-find new Pokémon that they didn't even know existed until they heard stories about about them, to try to force moments like the "there's a Mew under the truck" rumor. But if so, I think that they way overshot this mark.
If there were no new pokemon in the last 3rd of the game players would have been pissed off.
They still could've had a few new Gen 2 Pokémon show up in the later part of the game without restricting them from the earlier parts as severely as they did. Just space them out more evenly.
Honestly, I don't think that the later parts of games should have a ton of new Pokémon, because then you're wondering where this thing was for the last 7 badges, and why you should add it to your team over some other Pokémon that you've already built up a connection to. Just evolved forms of previously-encountered Pokémon would be fine toward the end of a game.
And as for restricting a generation's new Pokémon only to the postgame, in general, I think that's a terrible thing to do. For any generation.
- Postgame legendaries for you to go find? Sure.
- Extra postgame Pokémon from outside of this generation's pokédex, maybe from generations with hard-to-get Pokémon because of hardware scarcity / no remakes for them? A welcome addition.
- But run-of-the-mill Pokémon from a game's generation that would've been ordinary team members or memorable wild and trainer encounters, but now you can only find them after beating the league, making them complete footnotes to your journey, if not completely missable or forgettable? I don't like that. Postgame-exclusive normal-ass Pokémon encounters from that game's generation don't make me think, "ooh, good, extra content," it makes me think "wow, why couldn't we have had these before?" It feels like when a game releases with day-1 DLC, and it's obvious that the devs just held back some of the content that they'd made. There's not the same financial incentive in this case, but that just makes it feel dumber. Making their new generation feel smaller than it should've.
I'll concede that it's more difficult to say what the "postgame" really is in Gen 2, since Kanto had a whole second league for you to go through, so someone could say that stuff in Kanto is not
technically postgame... but your journey through Johto still obviously seems like the main body of the game.