McMann offered no resistance after she collapsed. Her left hand (side against the cage) was clutching her abdomen and her right hand was on the mat. Her head was completely undefended and Rousey was teeing off on it for all she was worth.
If two men were in the same predicament, there might have been a significant risk of brain damage, and there is no question a stop would have been in order. But from such a peculiar angle, they were purely arm punches. Rousey was getting no drive from her hips. I don't think there was any such risk from a woman's arm punches.
There is no question McMann was offering no intelligent defense, which was the standard Big John McCarthy set back in about UFC3. So the ultimate question, IMHO, is whether they establish a different threshold for the women.
So on the basis of the danger of permanent injury to the fighter, I think it was an early stop, but I don't think McMann left Herb and choice. Like she said in the post-fight interview, "I should have stood up sooner."