I think you are missing what is probably the biggest factor: they can be very expensive to run if successful.

If you create a popular API, people are going to find creative uses for it, and because they can, by definition, be automated, you can get rapid growth in traffic with not that many users.

There is a bit of a 'tragedy of the commons' that goes on, because the people writing apps that consume the API have no incentive to moderate their usage, or try to be efficient.

Since the company that is providing this API is paying for the resources to run it, they can quickly get very expensive. Unless there is a clear financial benefit for allowing it to continue, most companies will shut them down eventually.

Is it actually expensive, or is it just a result of everyone moving to cloud providers that charge an arm and a leg for performance equivalent to a cheap laptop (when you account for "CPU credits" and all that) and try to nickel & dime you on everything, including bandwidth (despite bare-metal providers somehow being able to stay in business by offering unmetered 1Gbps bandwidth)?

Also the API forms a committed interface you are stuck with. Whatever way you want to "improve" your offering you have to make sure it's compatible with the old API, at least for a while. Especially as you can't reach your consumers.

Couldn't this all be solved with a p2p network that hosts restful services?

The framing of open APIs disappearing because of bad actors doesn't ring true to me. In my view, the golden age of APIs disappeared one-by-one as tech companies realized they won their respective markets and consolidated their power.

I think what you said, yes, but also bad actors. Basically they disappeared because what business purpose did they serve, they were a cost center not a profit center - bad actors just increase the cost.