70, 50, even 35 mil ppl may not want this form factor, so if we're counting different models that play the same gamecard (crucial aspect) then it should be wildly popular...up to 100 mil.
imo The key is to (eventualy) have multiple devices stacking the shelves, and give ppl the choice to what fits their lifestyle, be it switch OG, switch xl, smaller HH, mini pc design etc. These are all Nintendo devices that play the same game card - and if communicated clearly, again, will be a smash hit.
As for this particular model alone, I think the worst case is beating GCN lifetime sales, maybe N64. But I'm anticipating more models that play the card, so ... will be an interesting new era for Ninty.