BOTW Calendar Year Sales:
2017: 6,700
2018: 4,980 (-25.7%)
2019: 4,660 (-6.4%)
2020: 1,070 (-1.8% YoY so far)
Excluding the Jan-Mar quarter each year (this removes the impact of the huge launch quarter of 2,760). So Apr-Dec sales yearly:
2017: 3,940
2018: 3,200 (-18.8%)
2019: 3,570 (+11.6%)
There's no way this is going to go sub 25M - it doesn't make reasonable sense short of it falling off a cliff today.
In order for it to miss 20M this year, it would have to drop under 2,590 for the Apr-Dec period for 2020. That means a greater than 27.5% drop-off for the rest of the year despite the Jan-Mar period holding better than the past two years (its only down 1.8%). More realistically it should do another 3-3.5M for the remainder of the calendar year, putting it at 20.5-21.0M by Dec this year. Following that, it would have to fall under a 2M per year average from 2021-2024 in order to miss 30M. Missing 25M seems impossible.
Then there's an additional 1.61M from the Wii U version to account for (as of Dec 2017).
I think it crosses 25M by holiday 2021 and legs its way past 30M w/discounts/bundles/special editions/etc. There is no sense in a sequel cutting off its legs - that happens with GAAS/MP titles. A shiny new MK9 is a better option for a brand new Switch owner than MK8D, but a new Switch owner is not going to gravitate to BOTW2 before BOTW1 - it's not the same kind of experience.