Problem of Proton Drive is not only in the missing linux client. There is no support of WebDAV that makes integration with many open source tools (like note taking apps or photo management tools) almost impossible. Nextcloud is much better for me. Without a WebDAV (or one of other popular APIs) it looks like an attempt to create “walled garden” with vendor-lock on Proton, imo.
For me the problem is more in GPL violation: they distribute blobs under GPL3, user made a request of the source code by creating an issue, but they ignored that request. It is not only about “you have to fix it” versus “just fork it” imo.