Any app will not have access to the Kernel. Any new hardware installation with a different data link or a unique digital signal processing requires access to Kernel so that a new device type can be added to the I/O. Let's say a new electronic pod which is based on some new OS and a new data link needs to be integrated with the mission computer, first it has to be defined in as new device in I/O management of Kernel.
Just by installing device drivers will not optimize the functionality of any new device, it is prone to bugs, because there is radar, and other sensors on board which are managed by the Kernel.
If a Brahmos NG, or any Russian or French electronic Pod needs to be integrated into F35, the programmers need to create an interface by studying the device, why would French or Russians allow Americans to do that. Then there is a deadlock.