It always strikes me while watching TNG how inconsistent the development of medical technology seems to be: we frequently see people undergo drastic physical transformations through surgery, Picard has a fully-functioning artificial heart that rarely malfunctions, lifespans have been greatly extended with it being seemingly the norm to live past 100, and normal sickness appears to have been all but eliminated, at least within Starfleet and in affluent parts of the Federation...
I don't recall many instances of drastic physical transformation surgeries - but I recall they got Picard dressed up as a Romulan, which seems silly as most planets would have sensors to show different species about.
Picard's artificial heart is the sort of futuristic advancement that feels plausible. The number of times they wave a blue-glowing flashlight whose ray resembles season 5's title credits bit looked too much like idle fantasy and magic wand waving. A few times they would say "the spot will remain tender for a while" and thank Apollo regarding the knitter scene in "Contagion" between Dr. Pulaski and her nurse. (directed sonic waves stimulating rapid, controlled bone growth and without causing cancer seems borderline fantasy but at least they gave that device (never used on screen) a proper name.
On-screen in TNG, it's said on multiple occasions poverty is eliminated in the Federated worlds so there's some redundancy in your last sentence.
But then some stuff appears to have been changed very little. No major advances in birth control, it seems, given how often accidental children seem to pop up (you'd think at least the implant would be mainstream by then);
Given Roddenberry's known beliefs, unless they changed since that Inside Star Trek record release from 1976, I'm sure more episode would have discussed the issue - except the show was quick to figure out it should be for full families, with sex kept minimal. Shame that was after "Justice", which may also have been a catalyst in the show's change of heart on that topic... that said, enough episodes implied Riker was going at it more than Kirk ever had... so it's a bit of a cheat to pretend that practical solutions and mitigations did not exist. There is no herd immunity for syphilis, gonorrhea, herpes, HIV, NRS*, UBT**, etc.
* See the 1993 movie "Demolition Man" for more
** see the previous asterisk for more
it's hugely inconsistent how we can deal with disability (Geordi's VISOR allows a blind person to see, but there's nothing like a mechanical exoskeleton that could allow Worf full mobility after he's paralyzed, for example); and we see people die from injuries less severe than Picard suffered in his academy days all the time.
The VISOR was just a step away from becoming a protoBorg... I do recall that "The Masterpiece Society", despite otherwise being a masterpiece of contrived trash, had some excellent scenes directly involving Geordi that not only cover multiple issues both directly related to plot and tangential ones beautifully, but also remind how even crap episodes have standout set pieces. It takes a lot for any episode to be the truest of barrel-scrapings.
That said, the technology was always available at the speed of plot. Or redundant brain stems too. "Ethics" should have not going the magical ending route as Worf's recovery subplot was strong enough on its own.
As for dying of less severe injuries, it's still a matter of getting to a person in time regardless of severity, and while TNG shows a lot of advancements, every society has various lapses it would not have considered. It's been said the Aztecs - who developed ingenious ways to cultivate crops and other positive aspects, never thought of "the wheel" - at least in terms of assisting in mass transportation. Their art features circles but that's not quite the same thing. Of course, negative aspects also existed - that's getting beyond the scope of the ultimate point in that any society might not recognize multiple uses for advancements or other issues. So one could argue that, in universe, they hadn't thought of it in other ways. In terms of a constructed fictional show, the scriptwriters forgot for whatever reason.
I know the answer to pretty much any discrepancy is "because plot" or "because different writers," but is any of this addressed in any of the fiction?
Because different writers didn't care or didn't have time, or because of plot necessity, the end result is still a story that still has to work within a limited time frame and arcs and multi-part stories weren't considered (in general, or in relation to the story's comparatively parochial content. Worf's not going to get a three-part story to go through his plight. Picard does, but the Borg demolishing the entire Federation is sufficiently epic by comparison... even then, there are bound to be plot holes that would render an easier and faster outcome for the story - or no story at all - if implemented. Is the story entertaining enough? If so then the plot holes and nitpicks just seem easier to overlook. And that's up to the individuals watching the show based on their own individual tastes and expectations. Heck, I'm one of four people who prefers TBOBW part two over part one, because it's not a traditional plot that was set up well (part one) but went down an unexpected road to conclude that was fairly ingenious... also, thank Unix for that as they had sleep mode long before Windows implemented it for laptops, albeit in a different sort of way!
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