Nolan makes new things regularly. Cameron"s been staring at the same stupid blue jungle smurfs for over 25 years.

the final boss should have been the completely opaque abstraction of madrigal electromotive, since "they" were truly the only long-term antagonist remaining. an invisible, everpresent "entity" with unlimited funds and power, operating invisibly to surround and contain walt. no human representatives, no explosions or gunfire, just a completely different kind of existential noose tightening around his freedoms, disappearing his family, removing his hiding places, etc