CapnZapp
Legend
I'm talking about things like the Archmage NPC stat block. This works only for a quick NPC that way outlevels the heroes or some run of the mill bog-standard high-level mage, not as something that allows people like Gromph or Vizeran to showcase anything resembling epic power.I guess this would be ok for a Legendary Solo BBEG ability, but I definitely don't want PCs accessing it. I was statting up Runelord Karzoug in 5e a while back as a ca CR 24 BBEG for facing 5-6 level 20 PCs (supported by various CR 12-18 allies), and I don't recall lack of multiple Concentration being an issue, at least the way he's presented in the adventure. The main requirements to make him effective seemed to be Legendary Resistance & Actions (including casting as an LA), Magic Resistance, a good AC, and tons of hit points. The rules supported all that OK, though he came out a lot tougher than some similarly CR'd critters in the MM.
I don't want a humanoid Archmage to be turned to a combat monster a la 4e Solos; I just need high-level play to offer mechanisms that allow NPCs to outshine even high-level PC Wizards to show them there's more power to be had (even if only theoretically, as you say).