That's all fine and good, I don't care if you're S3 compatible or not....

I do care if I have to write my own API client for your storage backend. Or if you have examples to go off of. Backblaze doesn't seem to offer either for non-C++/Swift languages. Complete non-starter.

The, perhaps obvious, win of being S3 compatible is that you open the door to thousands of existing S3 clients already implemented in my different technologies, for free. And you get the developers who use them as customers.