We've been hearing about this for years in one way or another -- WoW's numbers are simply getting too large. The final solution to this is to squish all the item levels from all the previous content together to form a more compact progression. Ghostcrawler and other designers have been saying for a while now that this is coming up in the next expansion, so it shouldn't be a shock to anyone.
One of the key things to remember is what Ghostcrawler tweeted today:
@Drazzildab Exactly. If it takes you 6 seconds to kill a level 90 mob, it still will post squish.- Greg Street (@Ghostcrawler) September 18, 2013
Blizzard will be able to accomplish this by running a series of relatively complex algorithms to decrease all the numbers from each expansion/major tier. And while performing the squish on a large scale like WoW is complicated, the squish on a small scale is easy to understand -- everything is decreased by some percent. There are areas in earlier content where the squish could negatively impact lower level character's ability to run previous expansion content -- ie, a level 70 mage might find it more difficult to run level 60 dungeons. But as Ghostcrawler correctly notes, the change of power in the squish is most important to preserve at the latest tier of content.
However you look at it -- just remember and remind people that the squish will not stop solo content.