Standards docs usually use not just SHOULD, but MUST and MAY. :) We should (ha!) consider which of the principles are so important they "MUST NOT" be violated, and which are more suggestions ("SHOULD") or things that are permissible but not necessarily a majority case ("MAY"). This could help us zoom in on the key items.
Topic on Talk:Wikimedia Technical Committee/MediaWiki Platform Architecture Principles/Draft 9-2018
I agree, but I did not find anything that would warrant a strict MUST, not anything that could be reduced to a mere MAY. Do you have suggestions?
You suggested to have horizontal scalability as a MUST. But a LOT of things we do don't match that. Anything that's in MySQL doesn't. I don't think it's fair to call this a must.