I've never been vegan or vegetarian, but I do think anything that makes you more conscious of what you're eating is going to help, if only a little. Even switching from Big Macs to McDoubles is probably going to help a tiny bit. If you ditch McDonalds completely, it will have even more of an effect.
I also think there's a bit of peer pressure to lay low when it stops working, like when you run out of stored fuel. You're not going to hear about the eventual lethargy or brain fog. No one is going to post about that because it's admitting failure. It doesn't matter that it's a failure of the nutrition (or lack thereof) that you're taking in. It still feels like failure, and no one wants to go on and on about that.
Throw in a little "This needs to work because it's the right thing to do" and it's all success stories.