My belief is: Amazon/Google (and to a lesser extent, Azure) capture a tremendous amount of revenue by coupling known OSS infrastructure with a hosted cloud business. This is because they've been able to extend unrelated monopolies
What you're saying is (I think): Google contributes code + projects back to the OSS community and sponsors OSS organizations.
I think if we could see the actual numbers — if it were possible to see Google's cloud revenue with associated OSS contributions — we'd see that what they're actually giving "back" is tiny, and it's the equivalent of donating to charities.
Since we don't know how many people actually work on OSS at Google, we can maybe use this estimate: https://twitter.com/mjasay/status/960563592683667456
Assuming Google pays ~$250k salaries to all of those folks to work full time on other peoples' projects, they're spending about $500mm/year on OSS while making ~$2bn/yr on just Google cloud. And that's horribly optimistic, I doubt much of that money is going to projects Google doesn't control.
Google's original OSS projects are different, I tend to like them. But given Google's market power the actual effect is to commoditize all the things Google doesn't expect to make money on. Kubernetes is great. Mesosphere is increasingly irrelevant because of it. OSS projects from megacorps tend to have the same effect as price dumping. They can afford to spend more money making something an non-viable business than most companies can spend at all.