I think the 360 was the focal point of development because it was far more popular for much of the gen. As a result of this, it seemed like devs were just developing for the lowest common denominator.
You saw something different this gen with PS4 and XB1 (base models). Devs seemed to prioritize PS4 for development early on, and the XB1 version sometimes struggled to keep up. This is not developing for the lowest common denominator, this is prioritizing the most popular platform.
I think it's possible that the Pro does in fact get more support than the X, if it ends up being the popular console. Sony has said that the sales numbers for the Pro are pretty good already, they have a year+ lead on X, and they are $100 cheaper. And don't forget the overall popularity of PS4 this gen.
With that said, these machines are all x86 now. Does that mean that porting and scaling games is easier? Are we talking PC setting sliders here, or some rough equivalent here? I don't know, but effort, cost, and potential return will determine the outcome here.