Pinboard's long-term income is primarily from the archiving users. The storage and full-text search index would be even more expensive to host for free.
There are way better architectures to have an unsocial bookmarking service. Pinboard has the worst one and because its focus is to copy Delicious and make money. In other words, it creates the problem first, and gives users an expensive solution.
An open source solution that can kill Pinboard may be based on even DropBox. OSS community is good at solving real problems.
I don't think I ever have my browser not open. I use something like four different computers, not including mobile devices, and appreciate the central copy without having to think about syncing it. I don't worry about setting up scripts on my local machine or updating my local plain text copy so that it can be searched. For the archiving users, they don't have to manually save the files of each website they'd like to search later.
If you believe you can do bookmarking better and free, then do it. If it matches what I'm using Pinboard for, and is better (and doesn't use Dropbox), I'll switch.
I mean, a free Pinboard was a solution for me when I was looking for a good alternative of delicious, 2 years ago.
But it's not free. The owner claims that he'll keep maintaining forever but he will not when he lose a considerable percent of his customers.
And he'll. As I said, pinboard solves the wrong problem. An online bookmarking service can be structured in a way that will cost nothing. Couple of scripts that manipulate a text file on DropBox was the solution came to my mind in 3 seconds.
I don't expect Pinboard to be around forever, but it's useful enough while it's alive to be worth the money for me. When it's no longer useful, I'll still have a copy of my data to grep to my heart's content.
Kind of ironic, that I am still bookmarking this on Delicious.