If contributors/maintainers are not happy with what the small company does, they can fork the project (assuming a liberal license) and continue in their own way. Valkey is a good example (with an interesting twist of license dynamics where Redis can use Valkey code now, but not the other way around).
> We have built a world where it is often easiest to just use whatever a cloud provider offers
And, IMHO, this is the major problem in the dev community these days - we've become lazy and focused on nonsense ("pretty"/unusable UIs, web gymnastics, llm, "productivity" etc.). We didn't have problems in the past to fork or reimplement OSes (various BSD instances), compilers (gcc versions), databases (MariaDB), and so on. There are tons of geniuses around hacking on cool stuff, but, sadly, the loudness of various hipsters and evangelists limits their visibility.
> Those providers may not contribute back to the projects they turn into services, though, upsetting the smaller companies that are,
The significant contribution that these providers (AWS, et al.) make to these projects is often overlooked - free advertisement. If I can remember correctly, ElasticSearch got popular when AWS started to offer it as a service. Additionally, cloud providers usually contribute (by employing core developers, shipping patches or testing) to the kernel, gcc or jdk, from which these small companies benefit significantly. In contrast, they themselves could do none of this.
But it is easier to blame "big scary clouds" than to rethink your business model. Be honest, start closed; no one will touch that and no one will be standing in your way.