I often wondered why blizzard doesn't take advantage of the content people loved. Sure, new content and raids are fun but how much time would it really take to upgrade Kara to current gear levels.
I mean why in gods name wouldn't you do things like this, the Ony upgrade was great but even now its outdated by two expansions. The scarlet monastery was good but you didn't even have to redesign the dungeon, just leave them as they were.
Can you imagine if Apple decided to completely change the Iphone every release? Sure a few things are changed on each version but the core design, style, and functionality are all there and people still flock to get the latest version. Why is that, its because they have a good thing going.
I'm going to sound a bit like a broken record, so forgive me, but when we are working on updates to the game, we're looking at what works well for the ongoing development of it. Sometimes that means taking a look at previous content and updating, and sometimes it means leaving it as it is. Overall though, it's about where we want to take things next, what fits into the larger scheme of development, what can we devote resources to comfortably and still deliver the best possible experience we can in a reasonably solid time frame.
There are a lot of things that people keep bringing up that they'd like to see again, or revisit in a new and fresh way. We're not unaware of this. But, at the same time, we have to keep asking ourselves, "Does it make sense and if it does make sense, does it make sense right now?" In contrast, there are those detractors who are often upset when we do go back and refresh content. In some cases it's because they're new enough that they haven't seen it yet, in others it's because they feel we should leave well enough alone, and to others yet who feel that it's a development "cop out" and a mere recycling effort. That's not to say that these things aren't possibilities "someday", but what seems simple to you may not be quite so simple as all that.
An example of updating things on a very large scale, is Cataclysm. There was so much work done from the starting quests to the environments to the very flight paths, but to those who didn't go through the refreshed content, it was a "waste" of time. We also then had what many felt was far too little at the upper end of content for them. So, despite best efforts to refresh a significant amount of content, not everyone was pleased with it once the immediate blush was off the rose. What we must fight against is time, nostalgia, "What do I get to do/take part in?", or even "where's my challenge?" So, no, it's not as easy as that. In a perfect world, maybe it would be.