Unless those schools are substantially increasing media revenue theres still not necessarily a benefit to monopolizing if you're looking at it from the school perspective. You can monopolize the market and still reduce payouts per school. They're not mutually exclusive.
Each school is getting what 60 million currently per year? So $960,000,000 overall coming into the conference annually through the media rights deal including the LA schools.
Getting to 20 means you need to get the media rights contract up to $1,200,000,000. 24 means you need $1,440,000,000.
And thats just for each school already in the conference to make the same amount its already making now. As evidenced by the types of deals the big 12 just signed and the problems the pac 12 is having securing the deal with the teams being proposed, there is a finite amount of money out there to be had. I don't think adding that many mouths to feed necessarily increases the existing schools' bottom lines.
If the big 10 swallowed up the entirety of the big 12 tomorrow I'm not sure why the media rights for those schools would suddenly be valued at greater than 60,000,000 per school as part of the big 10 when they were worth 32,000,000 yesterday. Thats what a lot of this speculation sounds like. Just add schools a, b, c, d, e, f, g and h and suddenly over half a billion dollars more in media rights is just going to appear out of thin air.
At some point the only way you're definitely adding more schools and not watering down payouts is kicking out current members who take more than they add (looking at you Rutgers)... unless the school is Notre Dame.