You're actually (a) leasing usage of affordances provided by a system which someone else took the time to design and build, and (b) leasing hosting for your collection of bookmarks on someone else's system.
Why would you do that? Because you don't want to do any of these things: You don't want to spend time downloading and installing some bookmarking program, you don't want to design and build it yourself, you don't want to deal with storing data locally (even though it might be really a simple sqlite3 database).
Likely, you've rationalized you're lack of want through convenience, security, risk of data loss, someone else has done a better job at "solving bookmarking", etc. etc.
This is a line of reasoning which is perfectly valid. It's totally fine just subscribe to a service. However, it just so happens that your demand for such a service creates a market. And it's equally valid for others to charge you for what they offer.
Everything else is just opportunity cost calculations on your end. You don't want to fork over 22$? That's valid. But then you have to abide by whatever else the market is going to offer. If Pinboard ends up losing subscribers to Microsoft and sunsetting, then that has nothing to do with the quality of the service, but the very fact that potential customers are far too keen on compromising what they want, need and value in favour of the free, yet less-then-ideal offering from a corporate competitor.
The problem with "free" is that it's the very bottom of the barrel. It doesn't pay the operating costs of a service. The difference between Microsoft and Pinboard is that the former has the financial leverage to cover the costs of a "free" tier. It's this leverage that defines the edge when it comes to conquering markets, as it allows corporations such as Microsoft to set up a far more effective sales funnel. It just so happens that it also drives competitors out of a market, and stalls any form of innovation. As far as Microsoft is concerned, they have no need to build a Pinboard clone with feature parity, if customers are all too willing to abide with whatever OneNote allows them to do, even when that's less then ideal.
In short, there's simply no such thing as a free lunch. Even if the market tries to convince you otherwise. It's totally valid to feel that Microsoft is "horrible", but by the same token, customers eagerly relying on "free" services equally sets the bar for them to simply get away with "horrible".
