It wasn't only with Sliske's Endgame. We've experimented with replayable quests a number of times over the years, and using a number of different approaches. The results have always been very few players actually replaying, nowhere near enough to justify the additional work required to make quests replayable (in one experiment, barely breaking double digits). It is not ideal, but lower level quests are at least easy enough to reach on a new account. I wouldn't say it will never happen (we may experiment again at some point), but I also wouldn't suggest holding out any hope that it will.