How much does the switch have to sell to be a success?

[Soundwave]

If it could hold or only be slightly down from the 3DS that would be a success.

Expecting literal 3DS + Wii U sales doesn't make sense, because that isn't an actual audience. Lets be realistic, at least half (probably more) of Wii U owners have a 3DS too, so that leaves you with 7 million Wii U distinct owners at best, and not all of those people are coming back, not after the way the system tanked.

So there's probably in actuality like 3-5 million Wii U owners in play for Nintendo, not 14 million on top of the 3DS number.

Also it's simply harder to sell portable hardware these days (and hardware in general). You say if I walked 500km, that's a decline from 650km I walked the year prior, but if I was doing that walking uphill instead on a flat road, that makes a big difference.

With more and more cheapo tablets available this gen and more marketing for smartphone apps, it's harder to sell the same. The 3DS at least had a year or two where tablet were generally too expensive for kids and smartphone games didn't have more marketing than console games even. But today that isn't the case, Switch has tougher competetion than the 3DS did.