To start, this is not directed at any specific leak but a general concept. I have noticed that a vast majority of the forums seem convinced that Blizzard would never do another cataclysm on the grounds of "There was a lack of end game last time because of how much time was spent on leveling zones!"
With the new world scaling tech doesn't this kinda defeat that statement? They could redo all the zones and make them all scale from 1-1XX, thus making the old world content current world content for leveling pre-existing "max" level characters. No new zones needed as all the old zones would be "new" zones. World quests would provide good end game content and with how large Kalimdor and Eastern Kingdoms are there would be tons of room for adding World Quests over time not to mention it would be a good test for expanding scaling tech to completely revamped vanilla dungeons for 15(10)-1XX when queued in the dungeon finder, no scaling for manually entering then of course Heroic, Mythic, and Mythic+ for 1XX.
Back in they had a massive lack of "end game" content because, you are right, they spent a lot of time on low level stuff that had no value to max levels. Now they can make everything designed for lower levels actually be designed for any level from minimum to maximum! There is nothing stopping the zone reworks from being new current content. In fact, factoring in revamped vanilla dungeons and scaled leveling with the possibility of World Quests I feel like there would be more content than a traditional expansion before raids and that's not even taking into account the possibilities of new zones on top of that.
The only two real logical reasons I could see people arguing against another Cataclysm with Blizzard's modern tech is that they just don't want to lose old world content as the newer stuff would overwrite it, much like how upset people were when the original quests were lost and a lot of Tansmog and other vanilla items with them. The second reason being the idea of "I won't feel like my character is growing in power if everything scales" which has already "somewhat" been proven false in legion alone as we out scaled content with iLvL so quick they had to start scaling them slightly off of our iLvL. Thats not even taking into account say a lvl 10 in a zone with a limited skill set vs a lvl 110 in the same zone with pretty much their whole arsenal of abilities(I know I have a little bit of bias showing, I just don't understand why one shotting gray creatures 50 levels lower than you that have no value is important :S)
From a content and tech standpoint, how is there even an argument that there could be a content drought for end game with the new tech Blizz could utilize now compared to back in the day? From a nostalgia/fear of losing old rewards and power fantasy standpoint I can kinda get it but even then the claims of a lack of content revamping the old world now in days seems pretty far fetched.
Legion has done a very good job showing that level scaling content works, and legion has been by far one of the most content packed expansions to date! (I know the latter is subjective to opinion, but again I find it hard to see the viewpoint of there being a lack of content in Legion. Again, Personal Bias I guess)