I can definitely see that modular means different things to different folks. For me modular meant that each major chunk of the game was a module. Magic for example would be a module that could be removed and another completely different magic system could be inserted in its place.
That's how I saw it.
Like weapon. Many people thought 5e's weapons chart is too simplistic for anybody but the newest of players. Once you get some experience with the game the four weapons chart is just too basic. And even Wizards eventually admitted to it and that's why they added Weapon Master properties
But the fundamental weighted the game works with weapons is you roll at
to hit versus a
AC and if you hit you
deal damage to
hit points. Game is built on the
bolded items existing but you can always change the values of the
bolded items via modules. You can swap in a 4E system where every weapon has their own attack bonus and so every weapon is unique because they both have a damaged value and a attack bonus along with properties that makes every weapon
unique.
The problem is like what Mearls said
It's funny that the biggest pain point in the game - monsters - has the most instances of drawing too much from the rest of the system. For instance, monsters were not supposed to use proficiency based on their CR. They were instead supposed to have a skill rating that could change based on their story and nature.
The monsters kind of have fixed values and are tied to a module themselves when created so adjusting them with other modules on the player side creates issues because in a player side can do a lot with that modules. But on the monster side unless you designed them to do such a thing
Like he said Master should not have a proficiency bonus based on their CR. Monster should just have a ranking of what their good bonus and bad bonus are.
Like a CR2 monsters should have a total bonus for the attack rolls and their saving throws of about +5.
That could be a proficiency bonus of +2 and a strength bonus of +3 for some sort of animal. Oh I could be a proficiency bonus of +3 and a strength bonus of +2 for a knight's squire. Or it could be just a strength bonus of +5 via a zombified giant
But ultimately the problem This thread is based on is that spell casters get to attack monsters with spells that they get to add their proficiency bonus to and target monsters saving throws that they might not get to add their proficiency bonus.
But that's all besides the point
It became DC 10+ Prof mod + Ability mod vs d20+ Ability mod
That is a straight-up unfair roll. Legendary Resistance is a kludge to rebalance an unfair contest because there's no way to guarantee that you can drain all of the spellcasters slots in a Dungeon