Except that is sort of required. If you go to tackle new content, you need some way for your current gear to be outdated. You also need some kind of impetus for players to want to do quests. Experience Points, which grant levels, is the simplest way to drive players forward.
Levels are used for a variety of purposes. Keeping your Secondary Stat ratings balanced (without levels scaling back your secondary stats, they’ll keep climbing), limiting access to quests, limiting access to dungeons, limiting access to gear. There are many good reasons to add new levels for each expansion.
And since the survey is saying substantially fewer levels, we can look at anywhere from 60 to 90 being the new max level (using BfA as the example).