Is spell dmg worth at high levels?

[LudicSavant]

I would very much not consider those "single-target damage spells" for a wide range of reasons, most importantly that they're action-economy boosting spells and that they're damage spells that actually hit multiple targets (even if over a longer period of time). They're essentially buffs more than damage spells.

Would you also object that Fighters do not deal single target DPR because they can choose to attack different targets on different rounds?

You can re-label it whatever you want -- the fact of the matter is that these spells are effective at eliminating big individual targets via damage. They fill the role.

Likewise, things like Magic Missile and Scorching Ray and Blade of Disaster and Bigby's Hand and so forth are capable of hitting multiple creatures within the duration of a single spell, but clearly excel at the single target damage role.