Inspired by this question, I realized that it's actually possible to know how much unspent non-RingCT outputs exist (although we cannot know which are unspent) by checking the total amount converted into RingCT outputs against the coin supply. If it ever happens that all non-RingCT outputs are converted into RingCT outputs, shouldn't it be possible to prune all non-RingCT outputs from the blockchain?
Also as a side note, in order to promote such conversions, would it be possible to incentivize it in some ways, e.g., by allowing the fee for such conversions exceptionally small or even zero?