May be a bit presumptuous to do a sales comparison with the monster that was Wii fit but hopefully time will tell. Hopefully ring fit will have amazing legs so your comparison won't be fruitless.
I don't think Ring Fit can win, but there are clear signs that the product is liked by MANY, I could bet that Ring Fit can reach at least 50% of the sales of Wii Fit.
Current PB on Secret of Mana remake : 2h27 (2nd)
Strongest worldwide achievement on TGM : 1st European S13 rank
Fastest TGM3 MASTER in Europe : rank Master V in 5min10
Western record on TGM3 EASY : 1484
Current PB on Power Ranger (Game Gear) : 10min06 (World Record)
Non-geek activity : ThermalHungary