"How it should be done" according to whom? This might be a dumb question, but I had the impression you are speaking for a larger group of people in your initial post. I would like to understand the context better in which the proposed standard came to be.
Personally, I don't support the idea of an open-for-everything "mw.randomStuff" naming scheme. It's half a century that I'm actively working with code that contains the sequence "mw." literally thousands of times: https://codesearch.wmflabs.org/search/?q=%5Cbmw%5C.%5Cb. After all these years my expectation is that stuff is only put directly in the "mw." namespace when it is general purpose utility stuff. And people are even trying to reduce this.
I understand that "mw.ext." is not terribly different from using "mw." directly. Both are places for all kinds of unrelated random stuff. But I believe it is still useful to have both: "mw." exclusively for random stuff that is part of MediaWiki itself, and a different one for community code.
Kind regards Thiemo