My employer replaced all our XP computers with W7 last year. They only had single core CPUs and 256 megs of RAM.
As to your "everybody knew" that's bullshit. People in IT knew, you knew and I knew but George Sixpack never had a clue. He doesn't follow the tech journals and had no idea, it isn't like his computer came with a big red warning sticker saying "you will have repairs equal to half the cost or more of your computer in 2014". He had no warning whatever.
Now, I have an XP computer I use to sample LPs and cassettes. My W7 notebook has no burner and the other tower runs kubuntu, and Audacity lacks needed features that EAC has. I'll just unplug the modem when I turn it on but George has no clue.
Joe isn't irresponsible, he's ignorant, and his ignorance is the result of Microsoft's irresponsibility. Making the arbitrary decision to EOL the software long before the hardware dies was also irresponsible of Microsoft.
Buying Window rather than using Linux was irresponsible on the part of OEMs, who knew full well when XP would end. Had the computers been sold with Linux rather than Windows this problem would not have existed.
So, how long until most of that hardware fails, then?
From my experience, at least another 5 years. In the last 30 years the average computer I've had has lasted at least ten years but something usually fails by the time it's 15.
Vista was released in 2007, so there are XP computers only 7 years old. A seven year lifespan for an OS is ridiculously short. If MS can't afford to fix its bugs and design flaws longer than 7 years they should charge more for it.