I honestly think 50 mil+ is achievable. If we look at shipments so far:
2017: 7,33 mil (launched in april)
2018: 7,69 mil
2019 7,94 mil
Shipments have been INCREASING rather than decreasing and judging by Q1 of this year and the switch in general selling like nuts that'll probably continue for this year as well. Obviously that trend can't last forever, but that would still put it at 31 mil+ by the end of this year with the Switch probably only being around halfway in it's lifecycle. If it does ship 8 mil this year, 19 mil over the rest of the Switch's life seems quite possible to me.
If MK9 launches on Switch this will obviously be thrown off though, but to answer the bonus question I don't expect that.
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