The DM always does and always will have to do some of the designers' jobs - that's the nature of RPGs. Good suggestions and guidelines are welcome, of course, but let's not forget that players and DMs will inevitably come up with situations, encounters, actions, whatever, that the designers haven't foreseen.
The DM is inherently a world builder, storyteller, and arbitrator. Not a game designer. It is entirely possible to play (good) RPGs entirely with rules as written. And it's entirely possible for game designers to create rules that handle situations that they did not forsee.
With refluffing and reflavoring, there's a heck of a lot of stuff that can be handled by using the mechanics of 4E, game rules as written, that might not initially seem supported. In addition, an important thing to remember is that not everything done in a typical RPG session is a "game".
When the party talks to a local lord, and just roleplays everything out, that's not a game. Even though the DM is operating without rules guidance from a game designer, he is still not playing game designer, because there's no game there.
Depends on the end goal. If the primary end goal is to build a game that incorporates fantasy tropes and enables a group to play though exciting adventures, you can do that and have fun with a game that doesn't value mechanical balance.
It's
possible to have fun with just about anything. That doesn't mean it's a good, or not-broken game.
That's not broken. It's just not producing a game that conforms to your desires. Claiming that it requires "fixing" because it's broken isn't true.
OK, it should be obvious that every post on 5E comes with a big fat implied disclaimer of "If 5E is meant to be a game that I'll want to play, then..."
If WotC has given up on me, and what I desire, then I guess that's their prerogative. But that's not been what they've been saying.
It's broken
to me. It needs fixing to
appeal to me. That's all the weight any of our opinions on this can carry.
It requires modification to match your desires, just like I would have to modify a Honda bought at your typical dealer to do all of the weird off-road shenanigans that they pull on Top Gear. In both of our cases, we might have been better served choosing a product closer to our initial desires in the first place. But in neither case are we "fixing" something broken.
We're currently being sold on 5E as a game to appeal to fans of all past and present editions of D&D. That includes 4E fans like me who are accustomed to, and expect, solid balanced encounter rules. If it fails to do that successfully, then it is broken.
If I buy a Honda advertised to be great off-road, and it isn't, and I have to modify it myself, then yes, it was "broken". It did not live up to its expected performance.