The other part of the equation for me is that $1 / month seems like the correct price for a useful service. I think Pinboard has something like 20,000 users. If they are all paying $1 / month (I have no idea if they are), then it feels like a sustainable business.
Other than the archival for $25, I don't see what I'd pay for that my own bookmarks don't already get me.
And the archival itself is likely only useful for stuff other people have already archived anyways; in the rare instances I find something I definitely want to ensure I have a copy of, I can usually find a way to get a copy onto my local machine and NAS.
The other part of the equation for me is that $1 / month seems like the correct price for a useful service. I think Pinboard has something like 20,000 users. If they are all paying $1 / month (I have no idea if they are), then it feels like a sustainable business.