This is the thing.
It's not about whether it's a good or bad ride. I'm sure the Velocicoaster will be a great ride. But that doesn't mean it would be a great ride for Disney -- it's all about whether it fits the area and the market. As I've said, you really can't theme a large coaster like Velocicoaster to Disney standards without enclosing the whole thing in a box, which is neither feasible nor advisable. It would always seem out of place at the current WDW parks, both for how it would detract from the theming of the surrounding area (hard to suspend disbelief that you're on another planet at Pandora, for example, if there's a giant roller coaster over your head) and because of how the ride itself can really only have light theming. You're not going to feel like you're in a different location or immersed in any kind of story while on one.
Again, that's not a problem -- it can be a great ride without those things. It has nothing to do with the overall quality of the ride in a vacuum; it's all about the fit for the park. Disney could build a new gate that focused on coasters and do whatever they wanted there without any complaints from me, but I'm pretty sure that will never happen.