Infinite spawning seems like step backwards in design. Like, where is the conservation of mass? Drop capsules are cool as player can see how a new enemy gets there, but just spawning a robot on already cleared area would be frustrating and unfair.
Robots have one benefit comparing to any other enemies - they can lie dormant. You could spawn disabled robots all around level during level generation and have them become active when player passes in front of them or makes some noise (gun, bomb…).
Also there could be some machines that bring in new robots as needed. As long as player is aware where new robots can appear.