Wow, that's an unexpected take on Learning,
Jitter! I really like it
We all experienced it, I believe, questioning, at some point, which use some of that knowledge pushed upon us could possibly have. Sometimes it didn't, but even then it's down there, providing a kind of common ground to build, or not, different things.
Now I’ve made myself sad, although I tried to make the story somewhat uplifting even though the theme is depressing.
Now, don't be sad. Some knowledge would surely be lost*, but other things would be learned back, like boats construction, farming, using plants and, in this case, magic!
And also some (particularly the Swedes and Danes, but also Icelanders) still use old technologies, so it's supposed that they keep and teach then to the following generations.
Magic, alone, opens a whole Universe (or several Universes?) of new and exciting knowledge (that eventually may be combined with the ancient technologies), opening great opportunities for anyone that wants to... write fanfics!
* Unless the government (or someone else), decided to create and maintain a kind of Library to archive the "ancient" knowledge, a bit like Asimov's Foundation**. In the future, when the danger from trolls and beasts are under control and wider areas are recovered, all that information could return to current use.
** That also had it's own agenda, but that's another story... or another fanfic!)