On the servers I played on, barely anyone had any interest in doing old raids for anything but achievements, and even then, there was still very few interested. Most people didn't care because no loot upgrades dropped, so from my experience I can say the players who actually like to run the old raids are a very small minority. As such, telling the playerbase to run old content as a "challenge" isn't gonna cut it.
But here's the thing Rank, when I was leveling my Guardian, I didn't even pay attention to how many levels I had gained, it was a while before I noticed I hit lvl 3, and I didn't notice I hit lvl 5 till I logged off of him to make a Sylvari. Why? Because I was having fun, I didn't care about my leveling speed, I was enjoying the game too much. Oh ya, the summer challenges are just a cover for not making new content. They could at least man up and explain why the aren't adding new content at a reasonable rate, instead of telling players to run karazhan (which is soloable at lvl 85) for a "challenge".