I think that's what makes this even worse. The fact that we HAD 10 man specific instances and then they took it away. It wouldn't feel so crappy if we'd never known it to begin with. It makes them look lazier, emphasizes the fact that when you're in a 10 man version of an instance you think "this COULD have been a completely different and new zone but instead bc they're lazy I'm farming the same old content." It makes you feel like they've cut something whereas you wouldn't have noticed a lack of content implementation otherwise.
I wouldn't agree with this. If they went the route your suggesting we wouldn't magically have twice as much content (a whole new 10 man instance as well a whole new 25 every major content patch). In fact I bet there would be fewer 10/25 man raids becuase they have to spend a more time making up two new zones instead of one. Your suggestion would only work if they hired another team of people to work on 10 man raids. That doesn't even take in to account the extra artists they would need to make the art work for 2 separate raids. If you want to argue they have the money for that I'm sure they do but they could easily hire those people to do both 10/25mans.
Going this route kinda sucks for the players that like having better quality items than someone who only does 10 man raids. That's only a small portion of the population though. On blizzards end it must be a nightmare coming up with 4 tiers of loot that are all fully itemized and all have different art work. Instead they will only have to do two tiers of loot with this system. I would almost argue the fact that they can do more content this way.
One last thing to think about is if they did 10 mans separate from 25 mans most people wouldn't see 25 man content. How many people saw more than just the first or second boss in Sunwell. This was a big problem back in the 40 man days. As a developer you don't want to spend months of dev time on something only 2-3% of your population is going to see. Like I said before though it will all come down to their tuning and we'll see how that goes in the beta.