I think the new stat locks are set up for 5-7 round combats.
Most of the casters have 2 of the higher level attack spells 2/day and 1-2 control spells or non-mage-armor buffs at 2/day.Some even have an even higher level spell at 1/day. Then you have the non-spell Magicks.
This allows for 4-6 turns before they run on empty.
In the full slot original variant, you have the minimum 7 spell slot of your top 3 spell levels. And if youre high enough level, you can keep 3rd level slots viable with the purposely overturned fireball. That gives you up to 10 slots of high power to fire off.
The thing is, I don't think 5th edition D&D was designed to run 15 round combats constantly. Those were designed for boss battles.
My preference would for enemy caster to not be for boss battles only.