But the GBA games are also made for those who are still on the GBA, which did north of 80 million worldwide. There was still that transition period.
And Switch is getting good 3rd party announcements in 2017 too. All those Dragon Quest games, Minecraft, some more indies, those popular sports games and other stuff. Is Monster Hunter the only metric of a good 3rd party support for a Nintendo platform?
Not really. Typically, good 3rd party support involves more than just a few indie ports, a Just Dance port, and a Japanese 3rd party title here or there. But given that it's a Nintendo platform, nobody should expect 3rd parties to be too supportive unless Nintendo hands out its franchises (Fire Emblem Legends, Rabbids crossover, etc.).