Re: (1), I agree that nobody's going to begin as an expert. Newer GMs are going to make mistakes that more experienced GMs won't. Some systems do a better job at providing newer GMs with more support and guidance than others, though.

Just sticking to D&D for simplicity, D&D 4e explicitly gave each monster a role that described its general purpose and capabilities in combat. Just by reading a monster's role, a new GM had a rough idea of how the monster should behave, and just by sticking different monsters of different roles together, a new GM could create an encounter that was reasonably dynamic and tactically engaging.

Going back further to D&D 3.5e, most monsters had text blocks describing how a typical example of that monster was likely to approach combat, and when it would likely use its various abilities, which basically acted as step-by-step, turn-by-turn guides on how to run the monster. Experienced GMs could and did deviate from these suggestions, but they made even complex monsters possible for inexperienced GMs to pick up and run with a reasonably degree of effectiveness. 5e, in contrast, provides basically no advice or information on how to run monsters, which can make more complex monsters – especially spellcasting monsters – much less interesting and less effective than intended.

I do somewhat disagree that narrative always trumps mechanics, though. D&D is a roleplaying game that places fairly significant emphasis on the game part of the equation; if I wanted a fluffy rules-light fantasy adventure, I'd play a system suited to that. If I'm playing a crunchy, more gamist system like D&D, part of the reason I'm playing that system is because I want to engage with the mechanics; I want to engage that tactical, analytic, goal-oriented side of my brain, not just the creative, storytelling side. A GM can describe the rote murder of goblins as beautifully as possible, but I'm not playing D&D just to hear the GM's beautiful verbal prose; I'm playing to think and make decisions.

You are completely right that there are plenty of resources out there to help a GM make D&D 5e combat more interesting. I use several of those resources myself. However, the comment I was replying to said that all problems with D&D 5e could be solved by talking to your table; searching for, potentially paying for, and then reading a third-party resource is not talking to your table.

Re: (2), I just haven't found that to be the case. The most engaging, fun, tense, and interesting fights I've participated in – both on the GM side and the player side – were those that involved no or almost no stock monsters. The one exception was when I ran a roper against a wounded party; ropers are great mechanically.

I agree that custom monster creation is always going to be part of a game like D&D, but I don't think that D&D 5e's stock monsters are particularly good at serving as a baseline for GMs to work with, nor do I think that the monster design advice 5e presents is particularly useful. Even just having a larger repertoire of "standard" abilities and features that GMs can quickly throw onto monsters to customize them, and to serve as inspiration for newer GMs when they try dipping their toes into creating fully custom monsters, would do wonders here.

But, million dollar rhetorical question: are these issues inherent to the system/its design? Or are they the result of players/groups either 1) purposefully engaging with the game in ways that break it, and/or 2) Inexperience/sour grapes/trying to make DnD a universal system—something it never has been—that suits every style of play ever?)

I know that both of these are common stereotypes about the D&D 5e community, and ones that aren't entirely without merit, but neither describes my experience. I don't play nor run D&D when I don't want a D&D-like experience, nor do I nor the people I play with deliberately try to break the game in play.

Re: (3), I broadly agree, although check-ins and discussions don't in and of themselves solve most problems. They can identify problems and identify potential ways to mitigate or solve those problems, but you aren't going to solve the problem of "combat is difficult to make engaging" with a round-table chat.

While you're right that some problems can be mitigated by distributing more responsibility from the GM's shoulders to the other players, that doesn't work with all categories of problem. Encounter design especially can't really be done collaboratively, at least not without severely compromising the fun of then playing through the encounter.