Locking in people into a certain platform, like you do with the Mac, insures a degree of profitability for a console.  I bring up the 3DO because the licensing model they set up ended up offloading all the costs of the hardware on the manufacturer, while the standards company got a cut of the licensimng fees.  End result was the console was as expensive.  If you think this is just 3DO, and is a simple issue, you got the taste again with the PS3.  The manufacturers have to be able to recover costs down the road for their hardware investment.  In the case of Nintendo, freed of the high def arms race, they produced a marginal upgrade to the Gamecube, with a new feature they added, and did it for a profit. 

As far as "no first party" goes, who is first-party in your model?  You propose something like a PC model for this, and the closest thing to that is Microsoft with the PC.  Well Microsoft has found moneyhatting third-party is the way to go.  They have a few studios, but not a lot.  Think DVD player manufacturers here to.  There is no first-party in the area of DVD manufacturers.  What you find is with this universal standard (to remind again, that is what 3DO pushed for) content makers and hardware makers tend to separate as time goes on.

The market has expanded a lot.  Do you think it would expand under one uniformed standard for hardware?  Do you think Nintendo would of done the Wii and pushed motion-control if they didn't control the hardware?  It is because they did, just like the 3DS, that they push innovation in ways not normally done.  They are built on controlling the hardware to meet their ends regarding what they want to do as far as games.  And this approach has led to greater variety.

An issue you have with universal standards and one platform, is that one really goes and markets the platform itself, unless it is like Microsoft doing it.  Also, in large markets, you find more competitors jumping in, who would undermine what you say.  I have learned this trying to get a non-profit involved with promoting abstract strategy games.  The supporters of different abstract strategy games see themselves as only interested in themselves.  They will NOT work together to get all interested in things.

First, regarding a title produced and distribution, you do have third-party companies that have their own dev environments which enable them to create games and easily port them between platforms.  They then will look to also distribute digitally, over the Internet.  They get their games on platforms that are not compatible, without worrying about the environments, because the dev work does it.

In regards to playing whomever, besides it offering console makers a competitive advantage, it would take the likes of a Blizzard to strong-arm the console makers into agreeing to allow World of Warcraft on all platforms they want to be on, to connect together.  Of course, this is presuming that there is a drive for EVERYONE to want to play everyone online.  There are more fundamental issues of screening out strangers who are jerks that isn't resolved.  No one has found a solution to this bit of human nature.  Myself, I don't really play online with others.  I don't have much of an interest other.  For me, I want to play with good people I know, who are friends.  Multiplayer boardgames is of greater appeal than online.  Play random strangers?  Well I do Game Room on my 360, and I get to play whenever for high score in challenges.  It works well.  Not sure how going beyond the 360 would help here though.

Hardware design is fully integrated into software design though, that's core to Nintendo and core to their recent resurgence.  Systems like DS or Wii wouldn't have happened with the scenario you're proposing, and it literally runs counter to Nintendo's own R&D.

People from Sega have commented before on how much turmoil their development went through in the 3rd party shift, Naka and Suzuki especially iirc, and how that loss of intimate know how with the hardware fundamentally damaged their R&D process and demoralized their teams.  When literally EVERY company who goes out of hardware "fails to adapt", maybe you should start questioning why exactly that is?

I would say another area that causes videogames to not be able to fit the model that other media like DVDs and music (and PCs to some extent) is that they have been history been evolving from a technical standpoint to be able to do more, get better, look better and improve.  With movies and music, the tech is pretty much set.  They are generally good enough for most people.  However, with videogames, we have had jumps.  Graphics get better, and AI and so on demand more.  The end result was the need to drive hardware to improve, and match the wishes of developers (you can see Nintendo's philosophy here). 

Post-crash (early 1980s) you saw Nintendo lay down a business model that was viable.  When Atari 2600 became the standard with all companies running 2600 games on their systems, there was a clear lack of quality control. caused the crash.

If Nintendo is to shift to this model, they're not going to lose that intimate knowledge of the hardware though.

Sure they will, unless this theoretical "games forum" allows them to design it.

Hey, a question.

Why is this a more practical idea than one hardware manufacturer just dominating completely?

I doubt it's more practical, but if one hardware manufacturer completely dominates the market, you hurt competition while still making people buy several platforms if they're interested in the exclusives, which then don't allow you to play those games with your friends, if don't own that platform.