If a sandbox is optional then it is not really a good sandbox
naturally even flatpak on Linux suffers from this as legacy software simply doesn’t have a concept of permission models and this cannot be bolted on after the fact
The containers are literally the "bolting on". You need to give the illusion of the software is running under a full OS but you can actually mount the system directories as read-only.
and you still need to mount volumes and add all sorts of holes in the sandbox for applications to work correctly and/or be useful
try to run gimp inside a container for example, you’ll have to give access to your ~/Pictures or whatever for it to be useful
Compared to some photo editing applications on android/iOS which can work without having filesystem access by getting the file through the OS file picker
naturally even flatpak on Linux suffers from this as legacy software simply doesn’t have a concept of permission models and this cannot be bolted on after the fact