It would be a bit unfortunate if all previously uploaded images “go dark”, in the sense that people won’t actually see them anymore.
This is true; but at the same time, viewers who do support both will show the same image twice.
Furthermore, we don’t tag for the renderer. Applications should learn to support ‘panoramax’, we should not duplicate data points to support some software.
I understand the pragmatism of your argument, but it will muddy the waters later on - especially because, if we don’t remove them now, we won’t know later on which images are the same.