> Also, how is a cowardly underachiever "the perfect protagonist for a teenage boy"?
It seems to have resonated a lot with male millennials at least. Many of my friends growing up loved Terry Pratchett. I loved those guys but calling them "cowardly underachievers" is probably fairly accurate, if a little mean.
All of the ones I kept in touch with have settled for a lot less than they probably could have done if they had been a bit braver. Few of them were even willing to move even an hour drive away from our hometown for better opportunities
They loved Terry Pratchett or they loved Rincewind? The amount of ways to do the former without the latter are high, and you seem to be jumping from a point about the latter to one about the former...
They loved his writing at least, I don't really know if they knew much about the man himself beyond that.
I think it's fair to assume that the loved his protagonist. They certainly loved how Pratchett played with tropes and often inverted them unexpectedly.
It's probably safe to assume that they loved Rincewind, since he represents a pretty big inversion of typical protagonist tropes.