I don't know how much this contributes to the success of Pinboard's one-time fee model.
It's possible Maciej structured it so that the one-time fee users bring in enough income to cover their own infrastructure and that the $25 annual covers just the extra needed to handle web archival.
But I also suspect he may depend on a certain percentage of one-time-fee users converting in order to cover all his infrastructure.
Maciej is very open about his infrastructure costs so perhaps he's already posted the answer to this and I just haven't seen it.