I realize this might be an issue but just straight giving them back isn't fair to people who thought about there might be value in saving them. Seriously. I'm told all the time about stuff I didn't "earn" and now folks want stuff back that they sold.
It's just a matter of practicality really.
Ideal might be a way to invent some method to earn it back for those who lost it through poor judgement. But how do you go about doing that without enabling people to earn 2 for instance? (which is clearly not intended). Whatever the way to make this happen it'd require a significant amount of development work.
Furthermore there was no expectation that "maybe someday it'll get *way* better" - until this announcement it had started to look like settlements were just going to become a dead and forgotten feature, like GvG. If the building was outdated in your city, selling it made some sense.
Personally I put them in storage if I removed them because it was a one-of-a-kind item. And I didn't even think about selling it because I have zero need for antique dealer currency and lots of other things I could sell instead if I did. But I'm not looking to punish those who did.