I agree to all of this. But the argument you "need" your concentration for Banishment or Stinking Cloud, so sorry fighter, you can just stand there and be a sack of hit points, is not a compelling argument for "the game doesn't work".
Now I can't remember if you made this claim, so I'm just saying.
For one thing, perchance the DM realizes no magic items makes the game harder, so perhaps you don't actually need to cast those concentration spells just as much as you're used to? Maybe in this campaign there actually is leeway to spend your concentration on your fighter buddy.
Maybe not, but there's a difference between "game is broken regardless of my choices" and "my choices is what breaks the game".
I think that the game's difficulty can vary wildly based on the choices of the people who play and run it. WotC gives us some guidelines for how it works best, but I've seen enough different experiences shared online to know that those guidelines only go so far.
I've heard the following claims made by different people. Now, I'm going to assume that nobody has some weird secret agenda here and is lying about what they've seen.
*D&D is easy mode, nobody can die ever or be challenged without house rules.
*D&D is busted if you don't force adventuring days with 6-8 encounters.
*D&D classes are balanced against each other.
*D&D classes aren't balanced against each other.
*D&D classes aren't balanced against each other...and that's a good thing because balance sucks!
*D&D casters are too good.
*D&D runs just fine in a low magic game without magic items.
*D&D has problems without the right magic/magic items and/or feats.
There's more, but you can see each of these opinions shared on any given day here on ENWorld. What often isn't shared, however, are the precise reasons these problems can or do not exist.
Generally this comes down to DM playstyle, player skill or lack thereof, character optimization or lack thereof, party optimization or lack thereof, presence or lack of optional rules modules, and what rules are used at the table.
I've now played 5e with three different DM's in home games, I've played under various DM's in Adventure League, I've played official modules, and third-party modules. I've played and GM'd D&D in general since 1989 or so, as well as other games (not making any claims of being an expert, just that this ain't my first rodeo).
My observations are as follows:
*Playing a martial when enemies that are resistant to your weapon attacks is miserable.
*Not being able to access decent elemental damage (like, weapon attack damage, not 1 fire from a torch or 1d6 from an alchemic item) as a martial, even with something that should exist, like fire arrows, is problematic against certain foes.
*Classes don't always have good ways to shore up the weaknesses of other party members. A Rogue being an ace at climbing does little to help a Wizard, nor does being good at sneaking help a Strength Fighter. Being able to cast Fly doesn't help the party navigate the third dimension, etc..
*Characters
do have glaring weak points that can only be alleviated by having another character with the right abilities on hand, or optional rules elements (feats, multiclassing, magic items).
*There do exist spells that can shore up weak points others have, by affecting groups of enemies, applying hard control, inflicting elemental damage, granting bonuses to bad saves, resistances, immunities, and the ability to undo the various terrifying things that can happen to you in D&D, like being turned to stone. But these often have opportunity costs and are only available to specific classes (Greater Restoration and Heroes' Feast are good examples, as is anything that requires concentration and locks you out of using other spells while maintaining them).
*There exists many magic items that can solve any and all of these problems, but their availability is completely in the DM's wheelhouse, and there's not a lot of real guidelines to tell a less experienced DM when/how/why they should make such solutions available to the players.
In another thread, someone pointed out that one of the reasons they rejected WotC's approach in 4e was because the game told you how to play- what classes to bring, and what your character was expected to do.
WotC was very careful not to do this with 5e, but I think they may have overcorrected. If none of these observations are true for you and the games you've played, fantastic! But they can be true for others, which is no reason to dismiss their existence, or resist the idea of WotC offering more guidance with regards to troubleshooting.
I mean, it's like when your make and model of car has a defect that only affects 7% of drivers. You may have never had this issue or noticed it, and odds are you never will. That's no reason for the car company to not offer a recall, is it?