I think it's fair to expect ongoing marketing, most notably TV commercials, similar to Ring Fit Adventure. Therefore longevity shouldn't be a concern and the game should have strong legs, especially because it's priced attractively at only $/€40.
Its sales curve should be similar to Ring Fit Adventure, initially even better because the production capacities won't be a hindrance. I'd also call it a strong possibility that Nintendo will sell a specific Switch + Switch Sports bundle in the not too distant future.
Given how Nintendo has kept talking about continuing to grow their Switch business, this game is a prime candidate to be a central point in their efforts. Many millions of people have very fond memories of Wii Sports and its sequel, so I'd be surprised if Switch Sports can't crack the 20m mark. Although I expect that it will take at least ten fiscal quarters to get there.
Lastly, to address a potential counterpoint - namely the low sales of Wii Sports Club on the Wii U - by now it should be clear to everyone that the Wii U was a piece of trash that wasn't wanted. Wii Fit U's low sales on the Wii U were considered by many to be a strong reason to be cautious about Ring Fit Adventure's chances to become a big hit and we already have the proof that that just isn't how it works.