A spellcaster at higher level doesn't have 15 minute days, ever. You have access to so many spell slots which allow you to address situations that most casters are better in most regards than a martial character. You just need to put a small amount of effort into magical resource management and think how much effort to I have to exert to get through this encounter with positive HP.
After all, health is really only relevant in combat at high level play since casters have many items and spells to buy them time to heal themselves and the rest of the party. Though, martials are definitely more consist, which makes having 1-3 in a party of 4-6 useful. The martial characters ensure that the casters can do their s&&$ so that the martials can do their s&*& better, it's simple. The only real way to stretch a caster's resources at higher levels is to make them participate in five or more combats a day, consistently, which means most of your days just consistent of combat.
I've started to appreciate mid level play more as I've participated in DND and Pathfinder games, simply because it feels like the sweet spot of balance between martials and casters that leads to less conversations about power discrepancies and more fun conversations about the game.