This seems somewhat similar to open source software, where there is also a free rider risk. This has not stopped large corporations from contributing to open source software, because there are often strategic advantages stemming from the software's existence and wide distribution.
I'm not sure how this applies to an investor funded rating agency. I don't know that there is any strategic advantage to large firms having accurate bond ratings publicly available, but it would be interesting to hear business models where there would be.
Normally there are transaction barriers that inhibit cooperation in providing public goods. However software has far fewer of those barriers. And so you get a situation where a network of loosely connected people each pursuing individual goals can collectively provision a public good.
I remember running across a footnote in The Logic of Collective Action saying that this was a theoretical possibility, but no example was known of it. (The book was written back in the 1960s.) In any case it is an extremely unusual example.
I'm not sure how this applies to an investor funded rating agency. I don't know that there is any strategic advantage to large firms having accurate bond ratings publicly available, but it would be interesting to hear business models where there would be.