I think Kosak and Metzen believe that they can't make any mistakes.
You would be wrong in this. I know that when you don't get to interact with them like I get to, you don't get to how deeply passionate and caring these two are, not to mention their teams. They don't work in a bubble. They pass around ideas, talk with each other and work out just how the story is going to evolve. It's a group effort and not just the work of one or two people.
The thing is, story isn't something that can simply be changed at the drop of a hat. While changes can be made to the smaller elements, it's not as easy to say "Ok, this whole story arc here? Scrap it." I'm not saying it can't happen at all, just that the very prospect has a rather significant impact on more than just story.
Think of it this way, major game mechanics can be tweaked slightly, but larger changes need more work and an opportune time to implement them. The same can be said for story arcs. There has to be a larger arc to work with and everything else gets built as framework to uphold it. Each of these smaller arcs can get minor changes here and there, but in the end, they're a part of the framework that's holding up that larger arc. Once you start shifting things around too much, you run into additional design issues that affect more than just the story. It takes extra time and resources to change. It's like moving support beams in a house.
An example of when we've made changes based on feedback and play testing would be Jade Forest. When we started the beta for Mists of Pandaria, the zone was quite different than it is now. So, to address some quest flow changes, we ended up making the zone inaccessible within the beta for a time to make changes to it to make the story and questing elements flow together better.
Add to that the fact that development is occurring well in advance of you ever getting to try it on the PTR and you can see that making changes isn't a quick and easy thing to do.
That said though, we are always always looking at constructive feedback. It's important. One well-structured post or idea (no matter where it's found) could be the impetus to changing the flow of the story in some small way or enhance it in another. That said though, we also do our best to preserve what we believe to be the core elements to the story. We have to stay true to it as well as we can and still push forward gameplay. We want story to enhance gameplay where it can and as much as we would love everyone to be enmeshed in the stories we're telling, we know not everyone is because it's not the thing they are looking for most. It's a balancing act and not a particularly easy one. Story is incredibly subjective and what may tug at one person's heartstrings or inspire them, will mean very little to someone else.
So, the best thing you can do is what you're doing here in this forum. Discussing what you like about the story, interesting new directions you think it will go or would like it to go, and you can also provide feedback from your experience on the PTR.
Story is very low in priority to the majority of Blizzard. Kosak and Metzen can have the greatest idea in the world but if it doesn't translate to gameplay that Stockton and GC like (both of which do not care about the lore the least bit) then it will never make it in game (GC for example is the one that cut the Warlock questline). It sucks but it's the reality of Blizzards internal structure and I doubt it's going to change any time soon.
It's worth noting that this too is largely untrue (minus the translating to gameplay portion). As I mention, gameplay is a major factor, but the story goes along with it and sometimes things get cut because they just aren't working well enough or aren't as cool as they could ultimately be. It's unfortunate, but that's what we work with on a daily basis. I've had many projects or items get scrapped because of other contributing factors. That's just the reality of things. We don't just do things because someone says it needs doing. We do things because they are (hopefully) the right things to do.