If XBox One gets totally trounced by the PS4 and declines largely from the 360, Microsoft will call it quits from dedicated gaming hardware and will take Halo and make it a multiplat brand like Minecraft. That said I think MS' future target will be Steam ... they will aim to make XBox Live an all-purpose Steam like service with streaming games as well.

Nintendo will kinda "cheat" this question, they will stop making traditional home consoles after Wii U, but Fusion will still have a home variant for people who simply prefer to play on a big screen TV with a controller (or want to play their handheld games at home and on the go) in the form of a micro-console (ala Vita TV).

Sony will make a traditional PS5, but even they are eying a back door exit from the business eventually with PS Now. The thing is I think Microsoft may have the last laugh here, as I think their online service will trump Sony's.

Microsoft has other areas of business that are far more profitable than gaming. The same can't be said for Sony or Nintendo.

I believe Nintendo will be the first one leaving because their console is doing pretty abysmal right now. Now, I'm not saying will pack up and go, but I do have the feeling that they will go mobile only, since it seems to be their only strength throughout the years.

This is going to get kind of hazy in the next generation though ... if you take a handheld chip, put it into a box (albiet a small one), plug that into a TV, and have a controller for it ... is that a console, even if it's just a handheld chip in a box? 

In that case Nintendo will likely have another "console", it just won't be a traditional console per se. 

I've heard about this from someone else in here. Do you mean a home console/handheld hybrid? Play console games in the house and play handheld outside with the same controller?

I see that being the route Nintendo go down. Mobile chipsets are getting so comparable to the dedicated home consoles now it's actually quite scary. If they can get a single chipset (much like Sony have done with Vita/PlayStation TV) and market it in 2 flavours but nail it, they'll be laughing.

If I was Nintendo I'd also be looking at partnering with a mobile phone manufacturer to make a Virtual Console phone. Virtual Console is Nintendo's way of going "multi format" without properly going multi format. Use releases on it to promote upcoming software and print lots of money from ROMs like Sega and Square-Enix have done with their back catalogue.

Keeping it all in a separate "Virtual Console" aspect would stop them losing control of it and they wouldn't have to pay royalties, which I suspect is the reason they have avoided this approach before.

No one will leave

All of them and none of theme.