I don't really understand the conclusion here. So the scenario is you trust dropbox with your files, and you trust them with a kernel blob implementing the filesystem, but you don't trust them to silently have accessibility rights?
You're assuming everyone trusts Dropbox with all their files and that everyone installs their kernel extension, which is a wrong assumption.
You can opt out of the kernel extension? Still, you give it root to install, and it has a long history of hacking the file browser to get icon overlays... it seems weird to me that this would be a deciding factor.