Let's say you're playing Gold or Silver (and Japanese encounter tables so no Route 46 Jigglypuff). You're playing entirely single-player, no trading, no events, no glitches. You're going to be missing the following Gen II Pokémon pre-League.
>6 starters
>Spinarak/Ariados or Ledyba/Ledian
>Pichu
>Cleffa
>Igglybuff
>Politoed
>Misdreavus
>Slowking
>Murkrow
>Gligar or Skarmory
>Steelix
>Scizor
>Sneasel
>Slugma/Magcargo
>Mantine or Delibird
>Houndour/Houndoom
>Kingdra
>Phanpy/Donphan or Teddiursa/Ursaring
>Porygon2
>Elekid
>Blissey
>Larvitar/Pupitar/Tyranitar
>Ho-Oh or Lugia
>Celebi
Which leaves 65/100 Johto Pokémon. Now if you actually were trading with an opposite version of equal progress, that number becomes 84/100. And of those 16 post-League Gen II Pokémon, only Murkrow, Slugma, and Houndour are "Kanto Pokémon". Their evolutions aren't found in the wild, you can evolve them anywhere, much like eggs are able to hatch anywhere, or trade evolutions can be done in any Pokémon Center. And Mt. Silver is a Johto location even if it's post-League.
But you also have to consider that there are people who play Pokémon who just hate engaging with most of the mechanics and spending time in optional areas, they just blitz through the game, finish the League, and never play again. Post-game is not content to them.
These people don't want to deal with
>Headbutting (Aipom, Pineco/Forretress, Heracross)
>Roamers (Raikou, Entei, Suicune)
>Swarms, backtracking, or 1% Pokémon (Dunsparce, Qwilfish, Yanma, Marill/Azumarill, Snubbull/Granbull, Remoraid/Octillery)
>Fishing (Chinchou/Lanturn, Corsola)
>Optional areas or content (Natu/Xatu, Bellossom, Sunkern/Sunflora, Unown, Dunsparce, Wobbuffet, Gligar/Skarmory, Smeargle, Tyrogue/Hitmontop)
>Friendship (Crobat, Togekiss, Espeon, Umbreon)
If I picked all the toppings/sauces/buns off my burger and ignored my fries and drink like a child, I'd be disappointed in my meal too.
