Running a Lumia 640 currently, I'm not directly affected by Microsoft's decision to only support newer devices. That said, I do have my old 925 running the preview that I have as a backup phone (and also a 635 that I picked up cheap a while back as my initial device for running early Insider Preview builds.) So I would like it if those were officially upgradeable to extend their useful lives as backup devices.
But I have to agree that Microsoft's messaging has been poor. From the beginning they should have made it clear that they were going to provide Insider builds for as many phones as possible and as W10M progressed, they'd incorporate feedback from that to decide which phones would ultimately get the official update. Even when they announced the final phones, they did it poorly in my opinion, leaving off the Icon, killing the idea of a 2nd wave, and then when questioned stating that they were still looking into it. Couldn't they just state something like at this point we're launching W10M officially for the following phones; we're still evaluating an eventual official release for the Icon (or for certain other phones if any others still have a chance.) The way they did it, it looked like they were waffling on their plans.
The other thing is I wonder - assuming Microsoft's stated reasoning about how they chose to cut phones based on insider feedback is accurate - how much divergence was there in expectations between the "insiders" and the Windows 10 team. I mean, if an insider voted that they wouldn't recommend a build because it was significantly slower than Windows 8.1, would they have voted the same way if they realized that it would possibly mean not getting an official update at all? (I assume that insiders were expecting Microsoft to keep optimizing W10M based on feedback....)
Would Windows Phone users rather have a compromised experience, but be able to run the latest apps written to the current SDKs? Here's an experience of mine from the Apple side of the fence. I have an iPad 2. Not an iPad Air 2, the old iPad 2. I picked it up on a deal when the iPad 3 launched; it was before the original Surface came out, maybe Spring 2012? Anyway, unlike the 1st gen iPad, the iPad 2 is still getting iOS updates and runs the latest version (v9.2.1 currently). However, it's gotten really slow with more recent versions of iOS, and Safari in particular seems a bit crashy, probably due to the small memory. Nowadays I don't use it for much of anything other than a way to keep my toe in the iOS waters, and also to run apps that aren't available on Windows Phone. Although I also now have an Android phone I switch between with my 640, so even that's less frequent. But anyway, if Apple asked me if I'd recommend running iOS 9.X on the iPad (or even iOS 8.x for that matter), and I was just comparing the experience from when I first got it running earlier versions, I'd have said no. But that also would have made it much less useful as a device now.