Adding 13 then would be nothing like adding 15 or 16 now. I suppose that is the core difference of opinion, and also truly unpredictable.

I didn't watch that 20 min video, so I don't know exactly what you're getting at.

Adding 13 created a dichotomy. The split grew to be quite harsh.

Adding a middle ground to the current dichotomy is entirely different than the act of creating the split in the first place.

It's not as simple as "adding a new rating is bad, because adding 13 turned out to be bad."

Adding a third option for non-kiddy movies would play out absolutely nothing like adding 13 and creating the split.

It would change the formula entirely. It would be nothing like anything prior. There are no valid previous examples of doing this.

The 13/R split is its own unique entity. It influences the biggest movie production business in a major way. 

A third option only helps the situation. While it isn't a big difference from 13 technically, it would create an opportunity for a new set of rules for a new rating. Every tiny amount of adult themes that can be added to a 13 movie will increase the satisfaction of mature audiences. Every small step towards less censorship and more realism attracts mature audiences.

Adults would be happier seeing a 16 than a 13.

16 year olds could actually see movies that are suited towards their interests without needing an adult.

And it's an easier jump for younger kids than a jump all the way up to R.

It's simply better than R in most cases. Actual R can be the new NC17.