I get that, but I don't even understand the desire to make leveling painfully slow anyway. But I do understand that too many people focus on "the next level" instead of furthering story or the adventure or all the other stuff that goes into the game. But painfully slow leveling doesn't make people not focus on the next level, it just makes people who like to focus on it uninterested in playing. I suppose there's some desire to purge the hobby of people who play in a way you don't like but that's just a disreputable attitude.
Totally tangential to the topic of the thread, but I found that in 3.5 and Pathfinder in particular, too fast leveling was a much bigger problem than too slow because characters have lots of abilities and tricks that become available at each level. If you gained a level too quickly you never really mastered the abilities you already had, which led to some wonky ability usage and even some sub-optimal choices the next level because certain synergies were not seen. I much prefer the "less is more" approach to 5E levels where generally characters only get 1 or 2 new abilities so it is not as big a deal trying the learn how everything works.