Imagine if Top Rank and Golden Boy merged.

this is correct. MMA fighters know this. I was watching an interview where one MMA fighter was saying "as soon as another organisation establishes itself as a legit competitor to UFC, our purses will skyrocket"

Though I agree with that, what will happen in the long run is that the sport will suffer from it. It would be better for UFC to straight up pay their fighters a decent wage for their worth to avoid this potential competition. Everyone would be happy.

This is what all the top pro leagues do be it Golf, Football, Tennis, Basketball, etc.

I think if Top Rank and Golden Boy merge, and continue to pay fighters a decent pay without getting too greedy, and over time either absorb or push out the smaller promoter, that would be good for boxing. All the top fighters would be under the same banner and therefore we would get to see more fights between top guys because their promoters would have less to lose.

Also, the promoter could spend money on promoting Boxing as a whole. Right now no promoter has an incentive to promote boxing as a whole because they would be spending their money to the benefit of their competitors as well.

Dana White can promote MMA as a sport because he knows that UFC has almost total control on the sport. Same way the NFL spends millions promoting football as a whole.

In fact having 1 promoter could even improve amateur boxing. As amateur boxing could be to that major promoter what college football is to the NFL. I'm sure the NFL spends top dollar to keep college football in the limelight and at a high level.