Magic Your Way. It's all about the Magic Your Way.
Disney can be impacted 1 of 2 ways by a unified effort between the combined forces of the Tampa/Orlando, non-Disney forces:
1. A reduction of locals. This impact is not catastrophic by any means, but I agree with jt that this is a definite to happen for an extended period of time, mainly after Potter opens, but Legoland will contribute to this. This will be more an annoyance to the Mouse (perhaps significantly), but on-property benchmarks wouldn't be impacted.
2. What would be potentially catastrophic would be an encroachment on Magic Your Way. Once vacationers have to choose between multi-day commitments, then that is where significant loss could occur. What Disney does not want to become (if even to a small minority of its base) is the second choice, as in "let's fit in a day at Disney if we can." (Kind of like we Disney folks try to fit it a day at Uni now.)
Everything, and I mean everything hinges on Magic Your Way. When those go, resorts, dining and per person spending in park go with it, because all of those things now are tied to the Magic Your Way ticket. Even resort stays now, with all the discounting and freebies ($750 gift card) are tied to the MYW.
It's all about the MYW. You want to kill the Mouse, kill the MYW.