I agree with your overall point, but...
"Does Nintendo really think they can compete with Atari, Magnavox, Intellivision, and Coleco with their upcoming 'NES'? Can they really elbow their way into this crowded market full of entrenched and experienced companies?"
It wasn't a crowded market. The North American video game industry had collapsed when the NES came out.
"Does Sony really think they can compete with Sega, Nintendo, NEC and Neo*Geo with their upcoming 'Playstation'? Can they really elbow their way into this crowded market full of entrenched and experienced companies?"
Sony entered the market at a time when Nintendo was both unpopular with developers (due to restrictive policies) and behind on hardware (due to sticking with cartridges). Sega was trying to come back from a series of hardware blunders when Sony undercut them by $100. The Neo Geo was never a serious competitor, and the TurboGrafx-16 only sold well in Japan. Sony is also a hardware company, and spent quite a lot of money ($1 billion?) to design custom hardware for the PlayStation.
"Does Microsoft really think they can compete with Sony, Nintendo, Sega, 3D0 and Atari with their upcoming 'Xbox'? Can they really elbow their way into this crowded market full of entrenched and experienced companies?"
3DO was from the previous decade. Not sure where you're getting Atari from. Microsoft basically threw monopoly money at the XBox, even buying Bungie so they could have a decent launch title. There were also lots of existing developers used to making games for a Microsoft platform (DirectX). Even then, the sales were pitiful next to the PS2, especially in Japan (which mattered more back then). The GameCube did about the same, and was the start of Nintendo's journey towards low-cost party game consoles. The XBox 360 was where Microsoft really got going.
Yeah, I think history says it can be done.
Under the right circumstances, with the right company, yes. And I think Valve has a serious shot at this. But it's not trivial. Half the companies you listed were utter failures in the market.
On the other hand, the market is different today. From the NES to the PS2, there was always a clear winner in sales and third-party support in each generation. But the latest generation was closer to a tie between the PS3 and X360 (please don't start talking about the Wii). We're now in a world where cross-platform games are the norm and two consoles can co-exist on equal footing. Hopefully there's enough room for Valve to push the industry in a different direction from where Sony and Microsoft want to take us.