The petition is open to all EU resident. The goal is to replace all Windows in all public institution in Europe with a sovereign GNU/Linux.
If the petition is successful it would be a huge step forward for GNU/Linux adoption.
The insecure parts of Linux is mostly on the DE side opposed to the core OS part that servers use. We absolutely will see more vulnerabilities in the future as Linux grows.
Many developments over the last few years have been for improving those aspects, e.g. Wayland is far more secure than X11 could ever be. There will be more vulnerabilities found, but it won’t be as bad as one might fear.
Wayland takes a lot of abuse but it is a great example of what is great about FOSS. Completely proprietary software could never abide that level of disruption.
If being driven by a minor player, it is just too hard and too risky. A commercial player with the economic dominance to pull it off would never see enough financial benefit to bother.
Take Windows. Even though modern Windows is from the “New Technology” branch of the Windows family, the security model was flawed with all users commonly running as Admin. Instead of really changing that, they have introduced a couple layers of duct tape ( eg. UAC ) but not fundamentally fixed it.