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GM Dave , those monster vault examples are a kind of micro modularity that is about swapping out things that all serve more or less the same purpose. You can make up new ones and put those in there because they are all about the same. Critically, if you want to do change the carrior crawler, you don't need to touch the basilisk, and vice versa.
The lethalness example is not like that. This is having a few concrete choices that have to be there on every creature to work. (Well, most of them need to be on most creatures. I suppose some creatures can just not be available for certain levels of lethalness.) But the main problem is that if you want to come up with your own lethalness "plug-in" to hook into those modules, you can't practically do it.
Table top games, in printed books, have been providing micro modularity ever since they started. Macro changes that work well are more rare. Sure, we've had reasonably constrained
lists (spells of a given level, classes available at start, monsters of a given power range), but those are of course handled by allowing the whole list or some subset of it. Adding to it is one thing; sometimes we might do that. Changing how everything in the list operates without having to rewrite the list, is another entirely.
So here's another test to determine if something is modular enough to work the way they say they want 5E to work: If you have to rewrite vast swaths of things to make a new option work, it isn't terribly modular. After all, if lists of lethalness options in monsters made 5E modular, you could just as easily argue that Basic through 4E were equally modular. I can rewrite all the monsters in those games to have my new lethalness mechanic, if I want. The only difference in those and having the options in the 5E manual is that the authors have done a few of the rewrites for you.
People that want to make the game their own do not want lists of pre-approved choices from the writers. They want tools to easily tweak the game into what they want without doing the rewrite.