Honestly, I don't know what the current XP-scaling logic is for the quests that supposedly scale to your level, but I feel like the scaling would make more sense if you did something like... I dunno:

qX(v) = ((10 - (v / 2)) / 100) * X(v + 1)

Where X(n) is the amount of XP required to reach that level from the level before it, v is the character's current level (as an offset within the expansion, so right now 80 would be 0 and 90 would be 10), and qX(n) is the amount of XP a scaled quest would give at level n.

So, basically, at 80, a side quest would give you 10% of the XP to reach level 81, meaning ten of the XP-scaled quests would get you there. At level 89, though, a side-quest would give you only roughly 5% of the XP to reach level 90, meaning you'd need about twenty of the XP-scaled quests to get from 89 to 90.

It would make side-quests viable to level a couple of alt-jobs, it would make them flexible enough to be used to level at different level ranges within an expansion, but running roulettes would still be more efficient. That said, I'm sure they've got more numbers on player quests and XP generation than I've got, so this is just spitballing an idea. But it feels like defining the XP for those quests as a percentage of the XP to get to the next level is a more viable (and flexible) method than just "base quest XP * <whatever>" or something.

That said, as it is now I did still manage to level one job to 90 mostly through side-quests and a few fates, and one or two roulettes... but I also had a food buff and a 15% FC XP buff going the entire time, for a grand total constant of 18% or more bonus XP at all times. (The "or more" being when the alt job fell behind the one I was leveling entirely through MSQ, so became subject to the armory bonus, or the places where rested XP was consumed.) That's not necessarily practical for everyone, nor is it sustainable indefinitely.