Redis is no longer free software

It is partly the model, and partly the context, I think. There are definitely sustainable open source projects. I would be surprised if you could predict their success from the exact model or licence, but I am quite confident that you could predict their success quite well from the context. By definition, a sustainable open source project has a broad base of unrelated contributors, so any model that doesn't support that won't work but I am not sure about the causal relationship. Models that encourage many developers and reduce the power of individuals or for-profit entities may simply be the outcome of sustainable projects that had no grand ideas about models.

As to why sustainable open source projects exist at large scale with commercial, for-profit contributors: the answer is pretty simple. It makes financial sense for them to contribute: if the software does not have a lot of added value (such as an in-memory database that your end users have never heard of) why pay $100 of developer time to reimplement it when all you need to is to pay $1 to fix a bug or $10 to add a new feature? When lots of contributors make the same decision, you have a sustainable open source project, although it's hard to see this making any one very rich. From the point of view of the Redis investors, they can't be worse off going proprietary, so why not?

Redis might turn out to be pretty generic and it might not be worth very much to people like me, who use it. You can run Windows servers in AWS, but who does? But I guess for the investors there is not much else to try, now that they have learnt that you can't beat AWS at hosting.