The reason for this has not really to do with bad design rather than expansion packs pushing the end game further up in levels. The end game likely would have been different if every character still was lvl 60 and all end game content was made for lvl 60 in mind.
Only way of solving this is at this point is if they make your character scale with the content so it's level appropriate. Plus, it would mean that they would have to give out proper end game rewards for doing that said content as well. Hopefully Blizzard will find a solution to this, since they have been talking about it for some time now. Frankly, I thought this would have been a good idea even back in TBC when it pretty much made Vanilla content obsolete.