As far as core rules being the default, isn't it more important to test the new rules in EA?
I would argue that is exactly what we've been doing and we point out the flaws in them. Also, Larian doesn't only implement new rules, to make them fit, they change and overwrite old ones.
Now, I'm not a member of the design team so I can only take a guess on the mind mapping evolution during design discussion. And I picture it going something like this:
-Alright team. We've been cleared to do this game. How do we move forward?
-well, we know what people have liked in our other games so we should build on that.
-well, in this fight with 5 goblins it would be cool to add an explosive barrel so that players feel smart hitting that taking out all five goblins in one strike.
-I understand you, but now the fight becomes too easy don't you think.
-well, we could add more enemies.
-ok. but now the barrel only takes out 5, what about the other 5?
-Well, there could be other tactical options like surfaces the players can interact with. like oil and grease and water that gets affected by spells.
-oh oh, I have an idea. with all the fire and oil around, wouldn't it be cool if that could help the melee players as well?
-ok. we include a dip mechanic but we are still talking low levels and spells that affect environment is higher up.
-well that can be changed by small changes made to the cantrips
-yeah, but another problem is that we seem to miss a lot, how do we change that?
-easy, we lower the AC.
-doesn't that makes the fight too easy again?
-well, lets raise the hp
-ok, that settled, how do we handle defense. This isn't real time so players won't be able to move away from enemies in an instant.
-Well, I read somewhere there is this disengage option. Wouldn't that work? And also, they can just jump/move away from the npc's during their turn.
-good idea, the problem is that disengage makes you lose the attack during that turn unless you're a rogue. so now we have everyone playing cat and mouse.
-can't we combine jump with disengage so that players still will get to attack each turn?
yeah, alright team I think we have something here!
-well, sir, doesn't this affect spell mechanics and class identity?
-Oh shut up Karen, can you for once stop with your negativity. We're making something cool here!
Now, I ofc am making exaggerations but what I'm getting at is that one change, leads to another change, that leads to two new changes, that quickly spirals out throughout a whole system. When all that extra work making changes to an already working combat system, could have been avoided if the first change wasn't made. A change made just to make one of Larian's darlings fit in.
I'm not saying d&d is perfect. that's why there have been several editions of it. and there is room for improvement without affecting the whole system. What Larian did with Rangers is a perfect example of that. You can bake chocolate chip cookies for people with gluten intolerance. But can you bake them changing the chocolate chips to gummi bears?