Flatpack etc sandbox is shoehorned into a system that doesn't expect it.
You can't expect apps to migrate to something that a) promises to support the existing model, and b) also support the existing model.
What these app package formats should be doing is coming up with a new model and then asking people to support their model as if it were a new platform.
Similar to how thunderbird supports Linux Mac and windows.
That might actually result in both an access model that makes sense to end users and actually creates an app package formats that isn't full of trade offs and work arounds.
You can't expect apps to migrate to something that a) promises to support the existing model, and b) also support the existing model.
What these app package formats should be doing is coming up with a new model and then asking people to support their model as if it were a new platform.
Similar to how thunderbird supports Linux Mac and windows.
That might actually result in both an access model that makes sense to end users and actually creates an app package formats that isn't full of trade offs and work arounds.