The MEV internalization comes from credibly ordering transactions by priority fee, so apps can rely on this as a building block and charge protocol fees to users based on their priority fee. You can think of it as "What if the apps could peek into the builder auction and identify high MEV transactions by who pays the most fees?" => the app + users win, searchers + builder lose. But builders can circumvent this mechanism if they have control over their ordering. That's why you lock the builder in a TEE and make them commit to using the exact ordering algo. So the key is to put builders in a TEE, not how many builders there are.