I suspect one thing that
@EzekielRaiden will find when he sits down to calculate is that aoe spells like fireball tend to do nearly as much as the concentration damage spells. So In practice the concentration issues are probably going to end up fairly negligible.
And I suspect those calculations will include encounters of 4+ creatures to maximize the AOE damage, despite encounters at high level not typically having that many creatures and if they do, not clumped up, and will fail to include resistances and saves against the AOE.
Looking at CR 17 and higher creatures in the MM.
Adult Gold Dragon: Immune to fire.
Adult Red Dragon: Immune to fire.
Androsphinx: No protections from fire or magic! We got one!!!!
Dragon Turtle: Immune to fire.
Balor: Immune to fire.
Ancient Brass Dragon: Immune to fire.
Ancient White Dragon: Legendary Resistance.
Pit Fiend: Immune to fire.
Ancient Black Dragon: Legendary Resistance.
Ancient Copper Dragon: Legendary Resistance.
Lich: Counterspell AND Legendary Resistance.
Solar: Magic Resistance.
Ancient Bronze Dragon: Legendary Resistance.
Ancient Green Dragon: Legendary Resistance.
Ancient Blue Dragon: Legendary Resistance.
Ancient Silver Dragon: Legendary Resistance.
Kraken: Underwater!!
Ancient Gold Dragon: Immune to fire.
Ancient Red Dragon: Immune to fire.
Tarrasque: Immune to fire.