De-emphasize combat? I suppose, but it is the core of the game, mechanically, and even the core of adventures.
In any edition, a player can make an ineffective combat-character. No one forces a player to make a combat-ready adventurer, even in 4e. You can pick feats that focus on skills, make non-combat stats the highest (or even just use lower stats than point buy gives), and choose magic items that are interesting in effect and flavor, if not the best in combat.
No rules are needed to make a character sub-optimal.
I would say is "has" been the core of the game so far, agreed, and I still want it to be there. But I disagree that it should be the core of the game, I think it should just be an aspect. That way it wont be the measure of every last character, which is what I felt 4e did.
Sure, I could have created a combat turkey in 4e if I really concentrated, but with 4e's focus on combat, that just made for a crap character. Thats not fun. What Im talking about is a way that I can create a character which is flavorful and doesnt get marginalized for it. For that to happen, the game needs to support you.
It funny that you said "No rules are needed to make a character sub-optimal" in the context that you did because it means you associate being optimal with combat capability. I dont, and I sincerely hope that 5e design takes that into consideration.