The argument that Nintendo will have a monopoly if the Vita were to die doesn't make sense.

[Mensrea]

The Vita isn't really taking much from the 3DS anyway. It's not like the market is split or anything close to that.

I think that if Sony chooses to stop producing handhelds, that Nintendo will have a slightly easier time, most notably in japan, but really it won't be that big of a deal.

I don't know that IOS and Android are really taking away from dedicated handhelds either. Think about the people that buy dedicated handhelds:

1. Young kids: They are not getting a phone, or even an ipod touch. IOS and Android don't really apply here.
2. Nintendo fans: Nintendo fans are buying dedicated handhelds, they are not going to abandon Nintendo franchises on the go for angry birds
3. Handheld gamers: There is a smaller group of people that really likes handhelds and buys them every gen. It's small, but they do exist, and I don't see them leaving
4. Japan: Japan seems to have no slowdown in terms of handheld sales, they love them, and probably always will.

The only demographic that I can think of really being lost is the older group who bought DS for brain training and whatnot, but look at GBA, a handheld that didn't have that audience: still had fantastic sales especially considering it's short lifespan.

TL;DR Nintendo essentially already has a monopoly, and iOS/Android aren't going to change that.