So, Ring Fit Adventure is following the trajectory we saw with many casual / mass market games during the Wii/DS era: Relatively slow start but strong sales for weeks afterwards due to word of mouth. According to Iwata back then, the casual games mostly started selling hardware around 6 weeks after their release if I remember correctly. That means Ring Fit Adventure could boost Switch hardware sales during the all-important holiday period.
And, as I've been saying since the Switch launched: Men aged 20 to 30 are not a big enough demographic to target. Nintendo needs children, women and older customers on board. That's why we are seeing Pokémon, Ring Fit, Brain Training and the Switch Lite being pushed by Nintendo so much. Only by catering to the mass market can the Switch sell in abundance of 100m units lifetime. And the Switch has the potential to do DS numbers. Nintendo just needs to play their cards right and make this an attractive device for mass market consumers.