I've wondered why we've seen resolution-obsession on Android etc devices, but not PCs. But there are some points to consider:Why restrict your speculation to decade-old technology, especially when there's already a host of gaming machines (iPads, iPhones, etc) with better than 96 dpi tech?
* Production costs may berelated to absolute number of pixels rather than dpi (which is a poor measure for saying one is better - e.g., my Galaxy Nexus has higher resolution and larger size than an iphone 4S, both of which are stats I prefer; the fact that the dpi is lower doesn't really mean anything to me).
* You need the GPU power to drive the larger number of pixels, and there's a higher standard of expectation on PCs, than "gaming machines" where Angry Birds is considered cutting edge.
* There's more pressure for certain companies to increase random specs in pursuit of marketing, whether or not there is practical use in doing so.
True, if we're talking of a hypothetical top of the range gaming machine, why not have a higher resolution with multiple monitors. But I don't think that Full HD is bad, or comparisons to cell phones and media players is necessarily entirely meaningful, and the OP did say "average" and "mid-high". And even if we think that companies should be increasing the resolutions, based on what they can do for other devices (just look at the Nexus 10's amazingly high resolution, for example), that doesn't mean they will for PCs. Personally I'm still waiting for netbooks to crawl past 1024x600...
I think a better thing to look at would be what's already available now for PCs, rather than phones - e.g., the 1440p monitor that someone linked to. SimonForsman makes a good point about making it easy to scale full HD content.
Retina is an Apple trademark, so something you won't be seeing anywhere else - it is simply their name for their resolutions, and not about any particular technology. As I say above, comparing on densities rather than actual resolutions doesn't make sense most the time (and indeed, the standard industry terms are to use terms to describe actual resolutions - e.g., 1080p, 4K).
"Retina" monitor: it'll be normal then!
Whilst 4k seems the obvious next step, I'd be surprised if this became dominant for computer screens within 3 years, given the average upgrade routes. Even for TVs, when I last checked a few months ago (in the UK at least), you have to spend quite a bit to get 4K, and many TVs (even say around the £1000 mark) are still 1080.
Frankly I'd be amazed if 1920x1080 lasted another three years as the dominant computer screen resolution. Computer resolutions almost invariably lead TV resolutions and people are already getting excited about 4k TVs.
Well in that sense, PC gamers (or PC gaming machines) are already a subset of most people, but I don't think that means they're going to stop being PC gamers - we've been hearing about how ipads will change everything for years from the media, but we're still waiting. If Android tablets etc (which are predicted to outsell the ipad platform) become larger than PCs, I don't think that means gaming PCs disappearing, anymore than it did with the growth of Android phones, consoles, or whatever. Also consider that tablets are only useful for some genres of games.This, or an iPad.
In 3 years time, a lot of people will have the same PC they have right now, so don't get too optimistic.
I'm more concerned about PC becoming less and less common than what silly specs it will have. Who needs 32GiB for a game? Few present games truly need something bigger than a single mid-class graphics card and few use the major part of memory on present day machines.
I mean, we might as well say than in 2015, people will instead have a Samsung Galaxy or a Wii U. An ipad is just one of many products. I doubt every tablet will be an ipad (or every console a Wii U, or every phone a Samsung Galaxy), nor will it mean that PC gamers stop buying PCs (they might buy a tablet as well as a PC, just any many have a PC as well as a phone and/or console).
Also consider that with hybrid PCs, the distinction becomes a bit meaningless - if ones "tablet" is actually something that is a PC too, it doesn't really matter
On a related note, desktop PCs are already less common than laptop PCs, and this trend will continue. But then, laptops are becoming more powerful as gaming machines too. So it might be that more gamers move to laptops, such that the average spec doesn't increase as far as one might think, but there is the increased advantage of mobility.