The proposal as of the time I'm writing this comment has 12 layers. That feels like too many. I'm concerned that absorbing and applying the knowledge of what layer a piece of code should exist in will be intimidating to newer developers. I'm also concerned that it will be a source of less-than-productive debate for experienced/opinionated developers.
Intuitively, 5-7 layers feels like the correct amount.
I am intentionally not suggesting which layers should be combined or what the new set of layers should be, because I don't think it is relevant. Yes, clearly we would have to determine that if we adjust the proposal. But my focus in this comment is strictly on my concern that the average human would have significantly more difficulty categorizing things into 12 buckets than 5-7, based on https://en.wikipedia.org/wiki/The_Magical_Number_Seven,_Plus_or_Minus_Two .
And yes, I realize the article I just linked allows up to 9 things. But given the overall complexity of MediaWiki, a person already has a lot crammed into their brain while doing any significant development on it. So I feel we should err on the lower side of that range.