The problem isn't the impact on FP at other attractions, the problem is the impact on walkways, length of lines for dining, shopping and Standby lines and lines for attractions without FP.
I don't think Disney will want to risk how negative of an impact this could have for the minimal benefits of adding FP. Not to mention that due to how the FP queue is configured, adding FP could cause some issues for guests that have sensory/crowd issues, right now it works as there aren't many people in the FP queue, but that queue could become quite crowded and loud.