I think that technically it's infinite but as you said, monsters don't respawn so the actual max will be limited by all the enemies in the world. I think that even the devs weren't sure themselves about the highest level that could be achieved and you have to remember that this is subject to change up until the actual release. So the answer is: unknown.
Obviously mods will be able to distort this in any number of ways and if Larian do expansions/DLC, those too will have an impact on the level cap.
