I suppose Mongo could sell exceptions to cloud companies, the way other companies dual license libraries or frameworks. But even Mongo’s bread and butter paid deals aren’t primarily about alternative license rights for open code. They’re about closed add-on code and services, as you describe.
Dual licensing, on its own, is an old and plenty good model for funding development of open source code. I’ve heard wind of dual licensing deals done decades and decades ago, maybe even before GPLv2.
The question is whether giants will pay the cost of reimplementing entire stacks, core and shell. I don’t have the time myself, so I’ll have to wait on a report about how compatible AWS DocumentDB really is.
Given AWS history, I’d expect they’ll get most of the popular functionality, most of the way, but gotchas will abound, and they’ll never hit 100%. Switching cost of code won’t bottom out unless DocumentDB takes lead mindshare, which closed clones rarely manage.
Successful open source does not require someone making money off developing it. It is successful when it is something that helps a profitable company but is not core to their business; then, they benefit from making it open source and having everyone contribute to its development and maintenance.
Or, you make money off support and consulting.
The key take away is, you aren't going to make money off selling licenses for open source. Which is good, I think.