I've long argued for applications to use file choosers they do not control, but not for security reasons. I've wanted it for UI reasons.

Somewhere I've got a screenshot of a Linux system I had with half a dozen applications trying to open a file. Each used a different GUI framework or widget toolkit, each of which had its own file dialogs, and so the user gets presented with half a dozen different file choosing UIs. It was completely ridiculous.

Making the UI suck less was not a good enough reason to get any of the major GUI frameworks to support separating file choosing from apps. Maybe improving security will be a good enough reason.