I know a lot of adult users who will immediately upgrade to RPi2, not really because of need but because its nifty and priced below the care-threshold. Same reason a lot of us bought the earlier version.
Speaking to friends about this its clear many previous Pi will probably just gather dust now. All would be happy to donate the previous one back to the foundation or an education setting but also admit, meh, its not likely anyone will actually get around to doing that.
Essentially people are generous, but lazy. I include myself.
What would make a difference is lowering the bar for donation. I'd like to suggest an easy way to reuse the packaging of a freshly delivered Pi to donate superseded units. Even just including a simple return address sticker would encourage some.
Speaking for myself, if at purchase of my RPi2 I could select an option to prepay return postage for my B I'd cheerfully do that. For plenty of people cost isn't the gating factor to generosity, time & effort are.
Sure you can also just ask for straight up donations of cash at the time of purchase but that's invoking something different. I think people feel differently about being asked for cash vs presented with the opportunity to do something useful with a physical thing they don't want to see wasted.
Am I wrong?