Out of curiosity, in theory can all android devices be rooted? It's my understanding that a root exploit needs to be found based on a vulnerability, and these vulnerabilities can be patched. So is it possible for a phone not to be able to be rooted?
Theoretically all Android systems can be rooted. Rooting is, basically, introducing an
su executable to the system, mostly done under recovery environment. The basic rooting process is simply putting an
su under a specific directory, while advanced rooting gives a complex
su executable as well as a GUI application for you to manage root permission management, to grant and to revoke. Besides, an engineering build of Android OS has root already enabled (stock su), so there shouldn't be an 'unrootable' Android OS.