Switch is in such a weird predicament. It is a handheld yet it isnt. Lol. Its definitely a home console 100% but its secondary functionality is basically another system (a handheld).

Did we call the ps2 a dvd player back in the day? :p

That said for OP its a success for nintendo only if it made enough software and profits off the thing. PSP/PS3 are close enough in terms of install base yet one of those things cost sony alot of money. 

Switch can sell 20mil and be very successful in one case scenrio and a failure with having sold 90mil in another

success ? 80 mil
good for nintendo ? 50 mil
not a failure ? 30 mil

How right this is... 

It's very sad that few gamers are interested in financial success or  some think that a super huge installbase gurantees much money making...

It's not if you sell the hardware on loss for long and it  can't be offset from software sales at least for some time.

A huge decline to 30 would of course be considered losing relevance, that would be like a 60% decline.

I was talking about a small decline, say 10-20%, would a drop to 65-70 million from 80 million be considered declining in relevance if most of their franchises see growth?

Well I was using an extreme to prove a point. If less people are buying more things, that's still losing relevance. If 100 people in a school own a Wii U, and 20 of them have Splatoon, it wouldn't be gaining relevance if 80 of those people end up owning a Switch, but now 30 people have Splatoon 2. The group of people with access to Splatoon has shrunk. Let's go further.

After that 70 people own a Switch 2, and 40 of them own Splatoon 3. Next, 60 people own a Switch 3, but hey, now Splatoon 4 has sold 50 in Hypothetical High. You see what I mean? The increase in software doesn't matter when it comes to relevance. The reach the franchise has does. A lower install base lowers its reach, which makes these things less relevant.

It will have to sell more than 3DS and Wii-U combined, so i would say 80-100 million. I already hear a lot of people talking about it, including casual gamers so it does seem to have a descent appeal.

It needs to sell more than the Wii and DS combined to be even considered notable. So 250 million.

But then it will just be a fluke, so we'll discount it.

To be a success I'd say as long as it's in the ballpark of the 3DS it's a success so at least 60m (aka +12m per year, since Nintendo is usually a 5 year cycle).

If Nintendo wants a hit then yes it needs to sell at least 80m (+16m per year).

It depend what kind of income can be emulate from the Switch sales. For the example, PS4 sales are very profitable thanks to it's PS+, and games sales and also third party developer games license. And also PS4 sales cost to develop is not that expensive that also result on better cost to sales revenue.

If Nintendo Switch want to be profitable then Amibo need to be on every games including third party games. And also online pay need to be on the switch. And also the cost and price of the Switch need to be balance.

Switch is a portable home console...

It's destined to  those people who want to  keep play what they  were playing on their tvs lying on the sofa, outside or in any other room inside their houses without a tv,  like toilet when natures calls them . Also, something that I have never  noted before , it's also made with regard to Japan and the not so great home console business state there. ;)

Nintendo cites that's it's home gaming system first and foremost.  So they want you to play it on home, on your tv  but provide you the ability to carry it anywhere and still play.

Look what Nintendo has killed... 3DS or wii u? hmmmm... Also,  It could be said that switch is portable robust  but cheaper version of wii u!

If switch can't be sold for 199 and less standalone ( albeit nintnedo could make a more affordable and protable redesign with lower specs ) , don't expect it to be aimed to 3DS audience...  Handheld console gamers weren't buying 3DS at 250$, they won't buy switch at this price too, even if it' two products into one, unless that appart from gaming on the go, they're interested in gaming  in home on tv  too...

So, i can't understand your surprise, believing that is for sure replacing 3DS is kinda arbitary since nothing corrobates this 100%... You can't just found it on the fact that  the hybrid seems more like a handheld console...

We have to wait a bit for any safe conclusions.