There's underestimating the popularity of a stocking filler and then there's shooting yourself in the face. Not to say Nintendo aren't capable of shooting themselves.

Nintendo's won the portable console wars why would they release another system requiring it's own software to compete with 3ds and Switch?

This is already the second such thread in the last couple of days. Since Switch is on track to do much better than the VGC consensus expected (30-35m lifetime), some people start to be inclined to move the goalposts in order to paint a negative picture for Switch. This 80m hurdle is amusing, but given how good Switch is, there's no reason to worry that Switch won't clear this new bar for success.

Beyond that, raw unit sales of console hardware as measurement for success can be misleading. For example, a GameCube (21m) is preferable to a PS3 (85m+) from a financial perspective. Some people say that unit sales are important to judge the strength of a brand and its future fortunes, but when both the GC and PS3 were followed up by consoles that sold/will sell more than 100m units, then it needs to be asked how much that really matters. The Wii U's pathetic 13.5m aren't stopping Switch either and Switch is clearly perceived as a home gaming system that can be taken on the go; otherwise the prices for software would have led to an outrage, because from a pure handheld perspective there was a jump from $40 to $60.

Lastly, we can look at Nintendo's stock price to get an idea where Nintendo is going. The link should take you to a graph that spans the timeframe of one year. The June 2016 level represents the value of the Wii U and 3DS. July sees the Pokémon Go spike in combination with the realisation that Nintendo won't get the full amount of the profits. September shows the response to the announcement of Super Mario Run, then the stock dips and hits a valley after the Switch teaser (mid-November) before recovering a bit for the launch of Super Mario Run. The launch of Super Mario Run sees many investors selling their shares because they can't see it getting any better than that level. The latter half of December through February represents the confidence in Switch; there's no enthusiasm, only a wait and see stance.

The period from Switch launch until today represents what Switch truly means for Nintendo: The smartphone business isn't anywhere near as profitable as assumed by investors and analysts, but Nintendo's core business is getting in good shape again. Unlike Pokémon Go and Super Mario Run, there's no sudden spike here, but rather a steady increase over time. This is a clear thumbs up from investors and they are right. Switch's trajectory will be one of beating forecasts, not missing them like the 3DS and Wii U did all too often.

In summary, the people who believe that Switch could be less successful than 3DS and Wii U combined can be brushed off as clueless.

Why is Nintendo no longer selling Gameboys? Your question is completely nonsensical.

It's not really about downing the price lots to compare it to handheld sales.

It's literally selling a unit as a handheld (without the dock) that's how you compare it to handheld sales.