I haven't seen people say that they will only have to focus on the Switch. Instead, I've seen it said they can finally unify the home and mobile software (which I've wanted and think will happen as well). That might mean dual-development for some time, or 3DS emulation, hard to say 4 months from release and 2 from real, legitimate information. Pokémon isn't showing how huge the 3DS is, it's showing how huge Pokémon is. It's the same reason Ingress was barely mentioned in the mainstream, yet Pokémon Go was (and still sort of is) a mobile juggernaut.
Take a look at some other games coming on the 3DS. Are people making a huge deal about them? Not really, in the cases of the 3DS versions of Super Mario Maker and Yoshi's Woolly World. Heck, maybe those two are examples of Nintendo's attempt at unification going forward.