But that just takes a bit of creativity with the light spell. Use the unseen servant to carry around a rock ( you can always put the rock in a box). Cast light on a sling bullet and then put the light in a fragile container.
Well first you've involved a second spell, which is a high cost. And the spell uses a first level spell slot, which is worse. You could cast it as a ritual, but then you need 10 minutes to cast it, and you'd need that 10 minutes every hour to keep it going. Second, you now have only one light rather than three to re-locate your foes with. Third, you've cut the range in HALF (60' vs 120') which really defeats a lot of the purpose of this strategy as you're now well within the range of darkvision of your foes.
If you're carrying around fragile containers in a dungeon that should last what, your first combat before they all break? Naw, they probably broke on the way to the dungeon
But how often are PCs with darkvision going to encounter opponents without darkvision?
You misunderstand. Of course your foes have Darkvisionn also.
Darkvision has limited range so you can find your foes while still outside most foes darkvision range with dancing lights (there will be some with 120' range, but not most). But that won't last long as your foes will try and close with you to try and see you with their darkvision. But their perception will have disadvantage due to it being treated like dim light while you are in darkness. Meanwhile, they will be treated as in bright light, as they will be in your dim light which is treated as bright light with darkvision.
The video by Treatmonk is quite good and lays out the impacts. The point is you can see them when they cannot see you at farther range, and at closer range they will have disadvantage to see you while you will have normal vision to see them.