I think people are overestimating the value of the GBC Pokémon games. They're obviously hugely popular, but I'm not sure there's a much smarter way to capitalize on them other than putting them on NSO.
Sure, they were best selling VC games on the 3DS, but that already happened once, which means the demand is far smaller this time around. By that time, those games had never been rereleased in any capacity. Now they are still available and have been for many years. Of course a lot of people who didn't own a 3DS would buy the games, and a lot of people would also buy them again, but I'm not sure those sales are worth it over the value Nintendo would generate by including them on NSO.
Now the gen 3 games are a differente story, obviously. But then, these would already be expansion pack only, which means it's even easier to generate revenue for TPC for their inclusion. I think A LOT of people would pull the trigger on the expansion pack with the addition of these games.
All in all, a NSO subscription including gens 1, 2, 3 with Pokémon Stadium 1 and 2 being compatible would be magnitudes more popular than it is now just by how huge the Pokémon fanbase is. I'm sure Nintendo knows that and they would definitely push for that to happen.