There is either consensus here that the Wizard, at high levels, is vastly superior to martials, or the people who believe so are just the most strident and incessant in expressing their opinions, but either way I don't see much pushback against that narrative.
However, I'm not so sure.
Some caveats for the following:
1) I recognize this doesn't address the complaint that casters get to do "cool things" while martials just get to make attack rolls. This is about the supposed difference in actual power/effectiveness in combat.
2) I have literally zero experience above level 15, so this only addresses tiers I to III
3) In the absence of magic items my argument would change, but while a goal of 5e was supposed to be that magic items are optional, I've never actually seen in played that way.
Here are my observations:
- First, the most powerful spells use saving throws, not attack rolls
- Monsters tend to make saving throws much more easily than they dodge weapon attacks (that is, than PC's miss with their weapon attacks)
- Far more magic items give bonuses to weapon attack rolls than to saving throw DCs
- More magic items boost Strength than Intelligence above 20
- Martials get advantage on attack far more frequently than monsters get disadvantage on saves
- Concentration prevents many of the best spells from being used simultaneously
- Casters have concentration broken fairly easily
- Two words: "legendary resistance".
- While many creatures have resistance/immunity to mundane weapons, resistance/immunity to magic weapons is very rare. Meanwhile, resistance/immunity to magical damage types is at least as common, if not more so, but can't be negated by picking up a magic wand (maybe it should).
What all this adds up to (again, in my experience, below tier IV) is that monsters too frequently make their saving throws, and casters end up contributing very little. And when they do contribute a lot it is not by themselves, but in synergy with a martial. For example, they banish the boss while the martials kill the minions. Or they haste the martial who then novas on the boss.
I asked myself: would I rather have a group of all martials, or a group of all casters? And except for some edge cases, in most battles I would rather have all martials. If you get extremely lucky on dice rolls a group of casters could win a tough fight, but it's far more likely that a couple monsters make their saving throws, they attack the casters who are trying to concentrate, and the whole thing turns into a rout. A group of martials is going to take a lot of damage, but they are also going to pump out a lot of damage, and overall have a better chance of winning. (Once again, my opinion.)
But of course what I really want is a mix of the two. Which kind of suggests the game is working as intended.