A duopoly isn't great, either.
You're right. Duopolies aren't great either and just because two companies are in fierce competition doesn't mean everything is good with the law. Both companies might be engaging in illegal anti-competitive behavior.
This needs to be discussed more in the age of AI. RealPage was just the eye-opener. Every industry that should be competing on prices has a cottage industry of "price recommenders" that is de-facto collusion in front of our eyes.
No one is saying the merger is illegal, just that it probably shouldn't be allowed.
If something is legal, the government should allow it.
I am saying that it shouldn't be allowed, because it is not in harmony with the laws, and is therefore illegal.