We have already made a grave mistake and destroyed the original game novelty of being able to only place avatars that you have unlocked, most players do not agree with all placement unlocked.
No good arguments exist in favor of re-lock something that adds creativity to one's map.
I highly doubt "most players do not agree with all placement unlocked." Everyone has benefited from being able to place a Zeus in their Greek Temples, robotics inside their sci-fi spaceships, and Sages in their fantasy adventure maps.
The NPCs aren't trophies. They are characters that add flavor to a world that cannot be achieved by having to physically unlock them.
I mean, you are in fact arguing about giving players "reachable goals" to obtain avatars, yet are quite content with the notion that players should be, for all intensive purposes, permanently forbidden to place certain NPCs. To have to beg someone who has unlocked the NPC to place it for them.
As if anyone can just shoot a developer or tester a message to drop by their map and place their exclusive NPCs every time one needs it.