This. I completely agree and i've been saying that for months. The moment Nintendo made an hybrid console it was obvious Nintendo abandoned the race for "unit sales" and instead entered the race for "profit sales" and this became more obvious after Nintendo announced their own Online paid service. That's why i not expecting Switch reaching Wii sales lifetime but still being the most succesful Nintendo ever thanks to 1st party sales and subcriptions for Online.
As for the rest. 18M+ is definitively possible, and it will be an enormous success because like you said, it will put ahead of PS4 in the same time period in shipment sales.
I still think Switch will fight to reach 17M shipped though.
That doesn't make sense, because Nintendo has definitively not given up on unit sales. The "sacrifice" they are making in unit sales this fiscal year will allow them to prolong the lifecycle of Switch, because they are pushing back price cuts and revisions to later dates. Nintendo is coming off the 3DS which had price cuts and revisions early; while that helped sales in the first half of the 3DS's lifecycle, it came at the expense of sales later in the lifecycle because the customers who wait for certain prices on the hardware were able to buy the console quite early.
Since so far Nintendo is doing things that will help Switch to have high yearly sales for a longer time, it's more than feasible to beat the Wii in the end, especially if they accomplish the goal of turning Switch into a one-per-person device as opposed to the one-per-household device it is right now.
The 3DS will have lifetime sales of ~75m, so Nintendo would have to begin to sabotage Switch in order to fall short of Wii sales.