From a combined 250 million sold units with the Wii/DS shrunk to barely 80 million with WiiU/3DS.

Will Switch really be a success with only 80 million lifetime sales, if it reaches that number at all, knowing it's the only Nintendo platform going forward? 80 million wouldn't have been a growth but stagnation over last gen.

Damn I don't know if the switch is going to reach 80 million, but that's a lot of lost market share

and what were the generations before Wii/DS?

It will probably not reach 80m, although it's certainly not impossible.

However, I don't think that's necessarily a bad thing. Maybe it's better to have one strong console with a lot of games, than 2 weaker consoles.

Although how many people who owned a Wii also owned a DS? It's not like that 250 million market share didn't overlap. But of course even then with Nintendo now only having one main piece of hardware 250 million is above and beyond an unrealistic expectation. 80 million is still achievable however for the Switch.

Most people that owned a Wii also owned a ds.

What determines a "success"? 

Anything over 40 million should be considered a success to me

I can't just count the console. The system has to also have a healthly amount of games. Which include 3rd party. Not all of them. But, key ones. Than i'll count it as a success.

Playstation and Xbox is installbase also shrunk from gen 7 to gen 8. Because Vita and PS4 are not going to sell 160 million + units. And the Xbox One is probably not going to beat the 360. Yet the PS4 is seen as a big succes and nobody cares about the Vita. Handheld and home consoles are two different kind of beats. If the Switch manages to sell 80 million I would consider it a successful console.