Grinding POTD/HoH and killing one single mob at level 1 are not the same thing at all. If you want to compare it correctly using mobs, it's like killing the same set of mobs over and over to level all the way up until 60, where you switch to a second group of mobs until 70. Of course, there's not a single legitimate leveling guide out there that tells you to farm level 1 mobs outside of your first class when at a very low level. You're not going to be grinding the level 1 mobs with alt classes the same way you do POTD or HoH.
The VAST majority of content you will unlock throughout the game is reachable on your first class/job. The only things an alt class unlocks is a different experience in content you already have unlocked (after a certain level when the class/job becomes interesting and only until it becomes a grind which leveling is) and class/job quests that not everyone finds enjoyable. When I did the Machinist quests up until 45, I found myself uninterested in the story of the Machinist job quests. Normally I find the story in this game enjoyable but not enough to grind out the quests for an okay story. (with some exceptions that don't include Machinist) Other than that, skipping to 70 doesn't skip any new content, just grinding on stuff you've already played.
My point still stands. You're eventually going to run out of classes to level in the lower areas of the game and will be eventually be at maximum level, or start a lower level alt character. And if there really was some other bit of content that you should to grind each class/job slowly to unlock slowly, you'll need to progress (actual progression) further in the game to get there. If being at a lower level gives you a dopamine rush, you're eventually going to need to be at max level to continue getting new content or get an alt to get that rush. And in the case where you do get an alt, my previous point applies.