For me at least, there is a major exception to #2 and #3: it doesn't matter how peripheral it is, or how many people use it, if it has a great educational value

For me at least, there is a major exception to #2 and #3: it doesn't matter how peripheral it is, or how many people use it, if it has a great educational value. Of course, prudence requires you to realize when you have extracted all the fruits of such educational projects. At that point, you need to stop and apply the knowledge elsewhere.