It's funny how much proponents of dropping support blindly believe what MS says, and often don't even know the technical backgrounds. An operating system is there to:
1. manage hardware
2. manage applications
3. provide a userinterface for those things
4. provide some common middleware which technically isn't part of the "OS", but via prebundling ensures high spread
Notice that only 1 and 2 may require a fundamentally different OS. 3 and 4 are basically just applications like everything else.
Now, lets look at microsofts history:
1. Generation: Misc DOS OSes. These mostly did only 1 and 2, so any new version would mean a fundamentally changed (though, only in a minor way) OS. Thing is, even here microsoft already made people reinstall the whole OS, even though technically replacing a few files of an existing OS (an "Update") would have been enough. It could still charge full price for it. But for marketing reasons, MS didn't do this - it needed to give you the illusion that a new version is an entirely new OS, which you can only use by completely replacing the past one. The good news: FULL BACKWARDS COMPATIBILITY inside this generation!
2. Generation: Windows before 9x. Technically was an application running on top of DOS. One could even install multiple versions of windows next to each other, and launch them like any other DOS-application. Here too microsoft needed to give you the illusion that installing a new version of this generation was a complete new OS, even though just a few files did change from version to version. Shit like registry and decentral storage of settings wasn't invented yet. The good news: FULL BACKWARDS COMPATIBILITY inside this generation!
3. Generation: Windows 9x (that is 95, 98, 98SE, ME). Here things started to go shit, but not too shitty. We get the registry, and microsoft pushes minor updates (mostly just regarding HW-support) as entirely new operating systems. In short, if you wanted certain system libraries to support newer hardware, you had to buy an entirely new OS, even though the OS would stay mostly the same and the difference would just be availability of some drivers, adapted syslibs and a different bootscreen. Win9x used tech that still relied on DOS, but it was abstracted away much more. BACKWARDS COMPATIBILITY inside the same generation began to suffer because of this, but still mostly remained okay.
4. Generation: Windows NT4. Completely new OS architecture that no longer relied on DOS. This is perhaps the biggest overhaul microsoft did so far. This indeed was a full completely new OS, created from scratch by MS and IBM - it was the only time MS ever did something like this.
4,5th Generation (2000, 2003, XP, Vista, Seven): Take NT4 and make it ready for something else than servers, add multimedia capabilities, good, comfortable but not bloated OS management software, and you get windows 2000 - perhaps the most sane OS-version MS ever did for its time. 2000 was a refined, polished and finished version of what NT4 started.
BUT, microsoft decided that it needs to give you the illusion that newer OSes are still needed - that they aren't "done" yet, and do not just require updates to stay compatible with new HW. So, they made a fancy new GUI-Style, added a hyperthreading patch (while not making the patch available for 2000), and sold it as.... XP. Some subsequent updates would only be made available for XP, even though they technically could run just fine on 2000. All with the goal to deceive you into believing, that XP actually is needed to do those things (when in fact, MS just didn't allow you to do it on 2000 out of their own choice).
BUT, the good news was: XP would stay the current OS for a long time, and be supported with updates for a long time. MS' sales didn't drop either. So, practically, you for the first time got an OS, that you could KEEP and adapt to newer tech via updates, instead of switching to a new OS for no reason.
But, this meant that in many cases YOU could decide which middleware and updates you'd accept. Thats not MS-style. MS had an agenda about which middleware (so, software that technically isn't part of the OS) it wants to force down your throat, no matter if you want it or not. So, MS took XP, added all that middleware, again made a new fancy GUI, added a few minor HW-patches, and only made the next directX version run on that new OS (even though, there technically was no need to do that) - and called that Vista - again giving you the illusion that this is an entirely new OS, even though it was just XP + new GUI + more middleware + exclusive minor hw patches + refusal to let DX10 run on XP.
Then, after they accumulated a truckload of hotfixes for Vista, they took this service pack, changed the default GUI-Theme a bit, added some confusing startmenu and explorer widgets - and called it Windows Seven.
So, just so you know, MS only ever created two completely new OSes - DOS and NT. Everything else was just applications, updates and artificial restrictions sold to you under the disguise of "new OSes". They could just as well charge for the individual kinds of updates and let you install those on your current OS. The only reason they don't do this, is because they don't want to let you choose. They want to decide which middleware and applications gets preloaded on your computer. Thats why you "need" to buy new "OSes".
Oh, and by the way, lets take a look at the competitors:
Linux: There is exactly one OS. Everything else are updates. And yet, it has no problem running modern hw.
Mac: Don't know the full story, but since OSX, there is only OSX - everything else is (charged) updates.
-----
So, for all those who claim that one cannot expect an N years old OS to run something, or that the same happened for 95/98/ME, etc - do your homework before making such claims, because they're false.
The only reason why people need Win7 for some future games, is because of DX10. The only reason for that in turn, is that MS artificially restricted DX10 from running on XP. And the only reason for that in turn, is that they want to decide which middleware is installed on your machine - it has nothing to do with the "OS".