1.) In 5E, if "martial" means "has no spellcasting or magical abilities in any way, shape or form," then there are hardly any martials in 5E.
True. 5 sub-classes out of 38 by my count, in the PH. Doesn't mean people who want play the concepts represented by those few sub-classes - rogues, pirates, assassins, scouts, theives, knights, champions, Robin-hood types, Conan-esque types, warriors, soldiers, fencing masters, gladiators, military commanders, and most protagonist Heroes in myth, literature, and the broader fantasy genre - should be /penalized/ for it.
Even if they are in as distinct a minority as the distribution of sub-classes might tend to imply.
Heck, especially if they're a distinct minority.
In short, 5E acknowledges the extra dimension of agency by making everyone a "caster", to use your terminology.
Not everyone, obviously. Or it would be 38 out of 38 sub-classes being casters. There are still clearly a few non-caster sub-classes in 5e, even if every class has at least one archetype that casts spells or has other supernatural powers.
My understanding is that 4E did a similar thing .. with AEDU powers
Close. It did create a sort of rough party between casters and non-casters via that structure. It also just plain devoted more space and development effort to martial classes. In the 4e PH1, there were 8 classes with 18 builds, and 4 of the 8 classes and 8 of the 18 builds were martial. Everyone had some degree of agency because of the AEDU structure.
But, while martial classes may have had the same number of exploits as arcane classes had spells, exploits were notably less versatile in the range of things they could do, and the variety of ways in which they might do them. Then there were rituals.
So 4e was merely not as bad as other editions of D&D, in that regard.
And, it maintained that rough parity for only 2 years, before Essentials came out with versions of the martial classes stripped of said AEDU-based agency.
5e is back to granting significant agency only to classes that use magic - typically by casting spells.
2.) Some people might object that third- and half-casters in 5E aren't really "casters" and don't have as much agency as "full casters," and that Paladins and Rangers and Eldritch Knights and Arcane Tricksters and Elemental Monks and Shadow Monks are still being punished with "less agency".
I'd certainly agree that they're really casters, and 'less' agency is still significantly more than the martial classes get. Not too worried about them, in general, since they also get most of the baseline mundane ability the corresponding martial types get.
The half-caster still has plenty of agency and choices even on the spellcasting tier. Which, incidentally, is why Eldritch Knights are the best kind of fighter.
OK, then.
I agree that from a party optimisation viewpoint it's better to have a party of casters (and I include the Paladin as a caster) than non-casters yes.
Where I disagree however are:
1. From a standard combat pillar standpoint you do not *need* casters at the upper echelons of play.
2. From a standard exploration pillar standpoint you also do not *need* casters at the upper echelons of play.
'Need' is relative, and campaigns can always be run 'tailored.' So if a party is sub-optimal, the DM just puts them up against challenges they can handle.
If that's your DM, party optimization is kinda moot, anyway.
So it depends what you are trying to argue, what I got from your original posts is that you feel you simply can't play non-casters and be effective. You can and no one will kick you out of the party. If you are trying to argue however that it's more optimal to play spell casters (Paladin included) then sure, I agree. I will caution you however that the gap between "Standard" and "Optimal" in 5e is much smaller than say in 3rd Edition.
It's a more nuanced point, I'm afraid. Of course you can play a Champion Fighter and be effective - an effective beatstick. You just lack the kind of versatility (and, in D&D, that always seems to correspond with resource management) that makes the decisions you make meaningful, and adds depth to the play experience - what someone started calling 'agency' at some point years ago.
Such DPR machines are fun for folks who like that sort of thing. The key is that not everyone who doesn't like that sort of thing, who does want an interesting character that provides agency and depth of play, wants to play nothing but casters. Martial archetypes should be open, as well.