The people who are complaining about old Pokémon only appearing in the post-game are a very, VERY tiny vocal minority.
Hoenn Pokédex (RS): 200 + 2 events at launch, 184 Pokémon only available in games released 1-2 years later
Sinnoh Pokédex (DP): 150 + 1 event available at launch, 338 + 4 events available in the post-game along with backwards compatibility
Unova Pokédex (BW): 153 + 3 events available at launch, +493 available in the post-game along with backwards compatibility
Kalos Pokédex (XY): 593 + 3 events, 125 Pokémon unavailable at launch due to requiring Poké Transporter
Alola Pokédex (SM): 300 + 2 events + 86 Island Scan available at launch, 416 stuck behind backwards compatibility in Bank which was not usable at launch
Galar Pokédex: 400 + 1 (Mew) available at launch, 489 stuck behind backwards compatibility in HOME which was not usable at launch
Kanto, Johto, Sinnoh, and Unova are the only Gens in the series that have access to their entire National Pokédexes (minus events) from launch. Hoenn, Kalos, Alola, Galar, and Paldea all released with arbitrary restrictions based on the later releases of other software. I think it's worth mentioning that transferring forward is also really boring as a method to obtain Pokémon. The 416 missing Pokémon in Sun and Moon don't really add to the game, they don't even have Pokédex entries because there's no National Pokédex in Alola onward.
