Yeah, I'm not saying it wasn't useful at the time, just that it's not useful now. As opposed to learning SQL or shell scripting which will probably be useful my entire career.
While the payoff of learning old > new is typically much higher (search: Taleb Lindy effect), I think matching your learning to the 'human api' is more important. For me, learning is very emotional: when I feel a sense of curiosity and intrinsic drive to know, I'll follow my nose, spend my time where it takes me. I want to spend this kind of energy in a certain way.
When I find myself with an instrumental cause or external need to know, it's most likely going to be because it's something typical, something old, in which case learning about it and being useful with it will require less of my spirit and drive to crack.
New language/tech fanatics tend to pressure through both sources (a la "look at our ingenious design breaking paradigms" and "it's so good your boss will want you to work in it (well, soon)"). Often the former argument is stronger, so it appeals to your curiosity - in which case you're best off searching for the useful kernel, followed by a swift exit in order to preserve your sense of discernment. Should you return there, provoked by your general interest, that ought to be your indicator of importance. How hyped you felt that one afternoon you learned about it after reading HN comments is likely not.
On the other hand, the 'should learn' brigade will tend to target the latter source of pressure (employability & centrality). If you find yourself feeling resistant to this and force yourself into it anyway, you'll easily burn out and douse your curiousity for the day. I've arranged learning resources to languages I find fundamentally dull multiple times, and made only very shallow dives into them.
When choosing what to learn, your built-in heuristics will tend to serve you much better than either a long list of common 'shoulds' or a tangle of overhyped 'musts'. Let natural forces do their thing in shaping what things will be presented to you: avoid paying much attention to the loud people where marketing, shills and zealots tend to roam.