I mean the fact that most people choose x7 over x1 or x3 means that they don't want the slow leveling...
Also x1 leveling on private servers is MUCH harder than it was on retail for the same game version since some quests with nice rewards just don't work. This definitely got better over the years on private 3.3.5/4.3 servers, but the mentality stuck with the playerbase that it's almost impossible to get to max level without grinding mobs for the last 2-3 levels. Heck a certain private server on 4.3 didn't have any Twilight highlands and most Uldum quests so you needed to grind the past to levels even on x5.
And also, the private server playerbase is the same as what people are stating to be the main playerbase of Classic, does that mean that most of them will just not even level to max?
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As stated before me, x7 is the modifier for XP and gold earned so players can focus more on endgame content.