It seems as though we haven't had an argument in favor for that side, yet.

Yes, it's called the OUYA!

Its going to happen one day regardless. It will be similar to Steam or Android platform. Where devices from different makers all log into the same service to game. We may see different services offered from competing companies and some of those companies may even be PC,tablet, cellphone makers themselves but it wont be the way it is today.

Yes, when everything goes digital, in terms of cloud streaming (not computing bs). It won't make sense to restrict your software, first party or third party to be distributed by one company. Their servers will be pretty much the same so I could see sony and nintendo allowing microsoft to host the servers, while the other two solely sell software. But that is 20 years from now.

Don't think a monopoly will happen, especially with PC in the mix.
If there were only one console, someone would soon come up with another console to compete with it.
It's the rules of capitalism

It will be the opposite. Games will be part of a streaming service and many more devices will be consoles. Sony will start by sticking Gaikai on every piece of Sony hardware and Microsoft won't be far behind with Windows and tablets.

I doubt it. There's plenty of room for multiple home consoles, and I don't see either Sony or Microsoft backing down or merging with Nintendo or one another anytime soon.

Doubt ... free market capitalism and and to some extent creative destruction would be at play. The weak die and the up and coming will take their place. In the event anything happened to the other participants and there was only one, I am inclined to think other companies would see an opportunity and try to take advantage of it.

it's the complete opposite, we are getting more and more devices to play our games