Not to be rude or objective here, but this is coming from someone who (
if memory serves) said that one of her major disappointments with B2W2 was that it had old Pokémon in it, unlike BW which was exclusively new Pokémon. Now don't get me wrong, I did like that (although it had to grow on me, because in 2013 it soured on me thanks to the B2W2 haters making it out to be the best thing ever), but if they had done that again with B2W2, then would it have the same special vibe to the adventure that BW had? If you already used the Pokémon present, then you'd be looking at the Pokémon you already entered the new region with - you would be pretty much playing the same game if it weren't for the old Pokémon that B2W2 had.
For that matter, let me ask you this: what about Platinum and HGSS? Or Emerald for that matter? The former extended the Sinnoh Pokédex (mostly with evolutions of the old Pokémon that weren't available in DP until post-game), so when it comes to remakes/third versions are you any more lenient with old selections?
It appears there's a misunderstanding here. Let's clear that up.
When a new generation begins, I expect a newer experience. That is why within that generation, I will use only the Pokemon that are native to it. (Unless it is a remake, because the remakes are harkening back to older generations.) However, the sequels to Black and White are within the same generation, so of course I'll want them to focus more on the Pokemon that were introduced in that generation. Another point I made in that post from many months ago:
This is a region completely distinct from all the others and that's why it has different Pokemon. It's far away, it's got a different vibe, and it's even based on the country where I live.
I was also disappointed in the fact that in Black and White, Unova was portrayed as being farther away, more distinctive from the other regions. By introducing older Pokemon, that separation vanishes, and the experience is ruined to me. I preferred it being more self-contained. Also, it didn't make much sense that there was so many change in the species in two years. That explanation was rather poor to me.
Plus, I hadn't even scratched the surface with all of the B/W Pokemon. Whilst I've used/seen a lot of Pokemon from Gen I, II, III, and IV over the years, I hadn't had the chance to endear myself to the vast amount of Gen V Pokemon. I hadn't looked at them enough for them to look familiar to me. They still look strange and new. A team of six is very limiting, and that's why when I played the sequels, I largely ignored the older Pokemon anyway, and formed a new team which has all Pokemon from Unova that I haven't used. (And one older one as per my rule and to reflect the change in the sequels.) It was still new to me because I didn't have a lot of time to get to know and see this generation's Pokemon. Whilst I've seen Eevee, Magnemite, etc. SO MANY times in my other playthroughs.
For remakes, by the way, I'll typically try to use only older ones to recreate that generation's experience better. So I will usually avoid using Pokemon with new evolutions all-together. Yes, you could say this contradicts what I said before about not wanting to play the same game over again in the form of a new game. Remakes are acceptable generally because I can't go back to older generations and back to those old mechanics once I've become used to newer ones. It becomes harder to play them. So having an older experience upgraded on a newer console is fine with me. Remakes aren't supposed to introduce anything new - they're intended to invoke nostalgia, so I generally remake my old teams (particularly in HGSS because of the Gen II incompatibility) to upgrade that experience. Although honestly, I'm not really too familiar with the original Red/Blue or Ruby/Sapphire, so the remakes were basically a newish experience to me anyway.
And I don't really see how a third version changes anything. I'll still use Pokemon introduced in that generation as the majority of my team. IF I decide to buy it, which I usually don't anyway. I typically ignore third versions because they don't have enough significant changes to warrant it. (Unlike Black and White 2, which had a new story and were sequels)
Speaking of older Pokemon who evolve into newer ones, those are acceptable because I'll be using the new evolution the majority of the playthrough anyway for the most part. I consider a new evolution both an extension of an old Pokemon and a new Pokemon in its own right. Similar to how I feel about Megas and why I would find using one of those to be acceptable.