Will it be possible to prune all non-RingCT outputs once we know they are all converted into RingCT outputs?

Question

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?

Answer 1

This would be possible only after every single one is spent. Given the extremely high likelihood that at least one output is lost (private key lost), it seems that it's unlikely to yield any benefit, though it seems it could be done per denominations, and there could plausibly be some denominations where all could be spent. Unfortunately, those are likely the ones with few outputs in the first place.