In 5e, that balance is upended because of that prior thought.
That balance concept was
already upended though. "Spotlight balance" sounds nice--everyone takes turns, everyone gets a slice of the pie. But it has two big holes:
1. The feeling of "doing nothing" when it isn't "your turn." There's a reason we make jokes about early-edition low-level Wizards throwing darts. It sucks to have to wait around to get some spotlight time.
2. The way D&D does magic, it's inherently opposed to
letting the spotlight rotate from one person to another. Because it's the (literally) magic "I win" button, which will
always steal the spotlight.
Spotlight balance works best in a game where the spotlight actually gets shared equally (contra 2), and where it gets shared
quickly, so you're never "waiting" for your turn. D&D hasn't been that game in a long, long time--if it ever
was that game. There's a reason high-level Fighters became landed nobility in ye olden dayse.
What should a fighter be, other than a character who is superior at fighting (it's almost a truism)?
So, a Cleric should be someone who primarily copies manuscripts? (There's a reason we call "written typos"
clerical errors.) And a Ranger should range? And, of course, suddenly
everyone is a Barbarian, because none of us speak Greek. And Bards should be historians.
The word is a title. Letting it
limit your design is asking for trouble. It should, instead,
inspire your design, provide a starting point rather than an inviolable boundary.
And yet, it would seem that spellcasters should have:
A. The ability to use cantrips every round that gives them roughly the equivalent of the martial character's attacks; and
B. The ability to use combat spells that aren't cantrips- from damage, to control, to buffs, to whatever, because that gives them variety in combat ... you know, meaningful choice; and
C. The ability to have numerous out-of-combat spells because spellcasting isn't just about combat, man, and that's what they have cantrips for anyway; and
D. The ability to use their numerous spells to affect the game in all of the pillars in a supernatural way, because ...
Right. You've seen how D&D magic--and this is something that goes all the way back, this isn't new--becomes the end-all, be-all because it's literally the "do anything" mechanic. There is nothing that magic cannot achieve, at least conceivably. Hell, magic is the only character option that literally allows you to
invent your own new mechanics. Fighters never had that option.
Which I totally understand. I, too, want my characters to be the awesome-est at everything! And yet, if spellcasters are required to be just as good as the martial characters at, um, martial stuff (combat), and spellcasters also get all the other goodies that spellcasters will get outside of combat, and if martials don't have that ... then, that kind of sucks for the martial, doesn't it?
Taking a moment to get the snark off my chest:
Congratulations, you have discovered the caster/martial disparity, and the fact that, while it's better than it was in other editions, it's still alive and well!
Okay. Snark over. More seriously: Yes. This is an extremely serious and thorny design problem. D&D fans, or at least a very significant population thereof, have come to expect a certain minimum amount of power, versatility, and comprehensiveness from the mechanical space called "magic." Put too many limits on that space, and people get EXTREMELY upset. Worse, there's
at least an extremely vocal, strident minority that get almost as upset if you give Fighters (and other "non-caster" classes) nice things. 4e tried to solve these problems, and got actively smear-campaigned as a result (often on completely false grounds, like "Fighters shoot lightning bolts out of their arses" or "there's no difference between a Fighter and a Wizard now.") 5e took the safe option of, more or less, rebuilding 3e with a few more power limiters (Concentration, fewer spells) and the most egregious offenders nerfed.
But it's still, as you've noted, an unsolved problem.
Important note, though: don't interpret it as "I must be the bestest EVAR!!" That's usually not what's going through people's minds. Usually, the problem is either:
A) Believing that magic simply IS better than not-magic, and thus magic-using classes SHOULD be better than non-magic using classes (uncommon in its explicit form, but often the
result of what many fans expect magic to be capable of)
B) Not really grokking how
limited they're forcing non-magic-using classes by restricting what "mundane" characters can do (most people don't realize just how
capable an actual Olympic-level athlete or highly-trained archer truly is, and fantasy characters are supposed to go
beyond those limits, at least a little)
C) Having dealt with the boredom of spotlight balance when you have few resources, and thus wanting to be sure they always have a meaningful contribution they can make, no matter what the party happens to be doing right now.
This, incidentally, is why I was so
upset during the playtest when they said something to the effect of, "If Fighter is 100% combat, Rogue is 80% combat and 60% utility, and Wizard is 50% combat 50% utility." The problem with this design concept is that, in practice, EVERYONE needs to be able to branch out and embrace EVERY pillar of play, otherwise you're going to end up with one-trick-ponies who can (potentially) even get shown up at their one and only trick.
And you're left with either giving martial spells (spell inflation), or not.
Well, there are really four options:
1. Nerf spellcasters until they're on the level of martials. This would piss off a lot of people and is thus not tenable.
2.
Actually make everyone spellcasters, so no one is left out. This would generate a
worse backlash than 4e got, and is thus not tenable.
3. Try (again) to nerf magic, the "I can violate the laws of reality" mechanic, while keeping martials bound by those rules. This has, demonstrably, not worked despite repeated efforts, and is thus not particularly tenable.
4.
Give martials something else. Something that isn't magic--something that magic
can't do--but that doesn't
nerf magic in the process.
You see examples of #4 in both 4e (where Martial characters got their own Powers, different from caster classes), and in the Pathfinder "Spheres of Might" alternate rules. SoM is a pretty much straight buff to martial characters, with all sorts of potent and unique talents mostly available only to non-casters; it also came after the Spheres of Power alternate rules, which HEAVILY nerf the versatility of casters while actually
increasing their baseline potency. (All Vancian spells are gone; spellcasters get a fixed, limited number of magic talents, which can each do one specific thing; talents are organized by the thematic/mechanical "sphere" they fit into, and require spell points to activate; spell points are overall MUCH more precious than spell slots are even in 5e.) You also see an example of #4 in early-edition D&D, where Fighters transcended their individual limits by becoming
lords and ladies, holding land, having retainers, collecting taxes, etc.: the Fighter "growing beyond" mere mundane fighting.
More or less, we can't go back to spotlight balance the way things are now. We either have to fundamentally change what "magic" means in D&D, and risk stoking caster fans' ire; or power up martials one way or another, with exactly the same risk; or accept that martials will never
quite keep up with casters, but we can put some bandaids on it and otherwise stem the bleeding so that, with enough DM elbow grease, we can hope nobody will notice. (Open secret:
people still notice. A lot. Hence this thread.)
And maybe I'm missing something on this, but the specific thoughts (that spellcasters had to be just as good as martials at combat) surprised me somewhat. Is it because everyone is playing 5e as a tactical combat game (I didn't think so)?
Nope. It's because combat is a "pillar" of the game, and people get bored when forced to endure stuff they don't contribute meaningfully to. But if you open that door even a smidgen, magic blows it wide open, because
that's literally how it's designed to behave: it breaks rules by making up its own new rules whenever it feels like.