The Crimson Binome
Hero
There is absolutely nothing which a spellcaster can bring to the table that is indispensable. At all. There never has been. There is always a mundane solution to every problem (short of specific world-building issues, where the DM fiat declares that magic is the only option).Further, because casters are the primary source of both indispensable healing and essential problem-solving tools, casters are the ones who set the pace of rests, meaning it is to everyone's advantage (whether caster or martial) to rest as often as possible in order to spend the most time in "peak" and the least time in "trough."
How do you propose to solve this problem that, up to now, WotC has twice tried to solve and failed--and which even 2e began to fall prey to?
Given that fact, and the truth that fighters can (and do, and have always been able to) demonstrate sufficient steady-state power, the obvious solution is to hold spellcasters accountable. Don't let spellcasters voluntarily opt out of their shortcomings. Remove mid-adventure spell recovery as an option.