Maybe i'm too optimistic, or i misunderstood the question but imo 100m is the floor, given the runaway success of the Switch and its successors' likely continuity. However, i noted that it's the highest possible option on the poll, so maybe a line of thinking may be that Nintendo somehow screws up? It's certainly possible but i just can't see it.
In answer to your bonus question, there's probably different answers for different posters/analysts, Going to go out on a limb and say anything over the LT of, say the NES (61m, or thereabouts) will be seen by most as a success imo. (Excluding the fact that Switch will sell LT multitudes more. If compared to the Switch possibly anything less than 100m will be considered a failure.)
DON'T WIN ME CHIBI BUDDY DON'T WIN ME.
ANIMAL CROSSING NEW LEAF FRIEND CODE:- 5129 1175 1029. MESSAGE ME.
ANDY MURRAY:- GRAND SLAM WINNER!
In my opinion the N64 was not just the best console of the 5th gen but, to this day the best console ever created!