You definitely made the right call but I don't think any research would have helped you on Sunday. I knew it was going to be busy but I wasn't prepared for what it was.
I, like you, had ALWAYS managed to find a spot even on crazy days. This one stands out because it was the first time I circled around the entire resort without any luck of getting in. I'm not "bummed" because I can go to Disneyland anytime, but I feel for the others who randomly showed up that day. And I definitely think it needs addressing.