Ring Fit Adventure has proven its longevity, so 2m lifetime are almost safe to predict now. It can of course go farther than that, but one factor to consider is whether or not Nintendo repeats the Wii Fit strategy of phasing out the original release and replacing it with a Plus version that would be counted separately. It's not likely that they'll do it again, because RFA was more robust to begin with and today's continuous updates via downloads allow Nintendo to add more to RFA without having to do a new release.
God I hope that's not the case. It'd be pretty lame if they came out with this popular fitness interface and only ever released one game for it. They'd make way more money and sell way more RF games by turning it into a whole fitness line with several games to the series. No reason to hold what could be a popular series back by only ever making one game for it, that'd be such a waste. It could be a 20+ million selling series if they made Ring Fit games a yearly release. Hell they made multiple LABO games, and Ring Fit is way more popular, plus with RF the customer only has to buy the hardware once, it would make no sense to not have multiple RF games.