Dana and the Fertittas are not idiots. They know that if they let Rousey fights at 140lb catch weight vs. Cyborg, she would get smashed, and they would lose their cash cow.
Think about it, if the Fertittas believe that Rousey can beat Cyborg, this fight would have happened a long time ago to further promote Rousey.
Actually no, they've been consistent about not letting anyone fight above their highest weight division.
Lesnar wanted to fight at super heavyweight instead of cutting 40+ pounds to make 265. He wanted to fight at catch rather than lose that much.
The Fertittas, who clearly wanted Lesnar (by far the biggest PPV draw in MMA history, twice what Rousey is now) forced him to cut to 265 because they didn't want people fighting above their heaviest division, and weren't willing to create a new division for one fighter, even a Brock Lesnar.
So if they weren't willing to create a SHW division for Brock - a much, much bigger draw than Cyborg, and a division with depth than WMMA 145 would be (Hunt, Brock, Tim Syliva all have to cut far more to make 265 than Cyborg does to make 135, even percentage wise), why in the world does anyone think they're going to make an exception for Rousey-Cyborg, which'll make half the PPV that Lesnar made?
People are living in a dream world, Cyborg isn't Brock, and yet they think the UFC will make an exception for her that they didn't make for Brock. Seriously, that's some extremely heavy wishful thinking going on.
This is bigger to the UFC than any one fighter, Lesnar or Rousey or Cyborg. They don't allow even their top draws to change the rules. Another example is their refusing to change their PED testing for GSP, who at the time was their biggest draw. Organizations don't change rules for any single fighter, no matter how big a draw.
The way to make this happen would be for Rousey to do what de la Hoya and Mayweather did, and start her own organization - and that's much harder to do in MMA than in boxing. Otherwise she (like Brock and GSP before her) have to follow the rules. People don't realize how little power individual fighters, even stars, have. Think of Couture's battle with them as well - again their biggest star at the time.