I don’t see a reason why they would go down that route.
Such a client would access a public API on one end and likely be a FUSE driver on the other one. Neither side needs to be “secret” nor does the client compete with anything.
However, even if it were not open source it would still provide a reliable fallback as it would always fulfill their requirements even when third party implementations temporarily might not.
They could also just contribute to this project to ensure it is always up to date with their policies.
I thought about that too but given how much this system is built around the browser it is most likely just a web application that looks like a file manager.
Thank you very much for the response and the links. The project shared by Astrada is particularly inspiring. The reason for my doubts about a possible Linux Google Drive client being closed-source is simple. As far as I know, the Google Drive clients on Windows and macOS are closed-source! So I don’t see much evidence that Google will stop being Google. But I hope you are right. However, I find it more realistic to place my hopes not in Google, but in free software supporters like you.