Another thing about Pinboard's killer feature: it's the kind of thing where, once you sign up for it, it doesn't make any sense to cancel it or restart it. That definitely doesn't hurt the economics any.
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.
Finally, there's one other aspect of Pinboard's pricing model: they offer a killer add-on feature (archival of bookmarked pages) which does have an annual fee.
But there are two other factors that may not apply as much to ThoughtStreams.
Several things have contributed to the success of Pinboard's pricing model.
Two of them have to do with the model itself.
Two services I use all the time but completely avoid the cumulative entanglement are Pinboard and ThoughtStreams.
With ThoughtStreams I paid to be an early user, got lucky and was rewarded with a lifetime account. Pinboard pricing has of course always been this way.
One big downside to monthly/annual plans is the cumulative entanglement factor.
Many of the kinds of people ThoughtStreams is aimed at probably already have several tiny subscriptions. Personally I have annual Flickr Pro membership, several domain name registrations, a VPN service, and monthly subscriptions/fees for Libsyn (podcast hosting), a small public radio membership, Spotify, Feedbin, and whatever else slips my mind at the moment.
After awhile you get wary of adding another little mouth to feed.