Things do not require to be useful or to increase revenue in order for them to be enjoyable. If the only reason you ever do something is because you get material wealth out of it, are you even making choices or are you a perfect rational actor as described in textbooks?
Things are allowed to exist and be enjoyed on the sole basis that they are enjoyable
Oh I’m not wealth motivated at all. My regrets are about uselessness of that time itself. E.g. I could learn ML instead and do nothing useful with it, rather than not doing nothing useful with my perl knowledge today.
I could enjoy one knowledge today without having monetary interest in it ever, but instead I have another knowledge which is completely useless even for enjoyment.
Then your contrarian phase ends and you regret that you didn’t learn something useful in that time.