Whether or not the Switch hits 100 million will depend on how long it lasts as Nintendo’s primary platform. I would personally assume it will average 20 million a year over the next 2 years. Taking that into account, I think the Switch *family* will hit the 100 million mark by holiday 2023.
I think the only thing that would keep this current iteration of Switch from hitting 100 million is if Nintendo releases a Switch 2 in 2021 or 2022. But even if Nintendo does do that, the Switch family/brand would still hit 100 million by 2023.