I'm not a huge fan of the isekai genre, but that doesn't mean I hate it either; its ubiquity is what makes me wonder why people don't just make straight up fantasy anime.
Frieren for example would be terrible if Frieren was actually some random person from earth who got sent into her setting (or, as Frieren's whole thing is outliving her comrades, her purple haired protege was the isekai'd protagonist); I understand how the isekai nature of an anime can also affect the anime, for example a character who has modern world knowledge he applies in the setting, or modern world tech, but if he just becomes a part of it, and NOTHING from his real life has any impact on the setting....then what's the point?
Overlord for example (which I didn't enjoy to be honest; I googled if he ever faces a challenge and nope, entire series and he has zero things that can challenge him) would not have been affected if he was just some lich who woke up a thousand/hundred years later.
Konosuba benefits from the isekai genre in that the isekai elements keep being relevant throughout (Spoilers:
spoiler
for example that guy who chose to reincarnate into the world with a powerful sword that Kazuma stole, or Kazuma dying and Aqua's replacement reviving him repeatedly, or that the demon lords at some point realized all their most annoying hero enemies keep starting in this one town and so decide to attack it, or that ancient scientist who turns out to be have been an isekai'd hero who creates stuff inspired by stuff he was a fan of
Most isekai animes today just seem to be wish fulfillment harem animes, which are a problem on their own as well, but they're paired with being isekais too (if you're wondering why I didn't make a thread on wish fulfillment harem animes, it's cause by and large I avoid them like the plague).
There are some interesting POST isekai stories that I found fascinating, stories where the characters came back from the isekai world and had their powers with them; in one case some of the heroes become devastatingly powerful villains who destroy entire cities, in another case it's a comedy about some guy who comes back and....could have been funnier without the ecchi/SA nonsense happening throughout.
Animes like SAO (which I didn't find interesting beyond the first story) make the other world an entirely false world and never let you forget the characters are real people who can actually die in the real world (sort of an anti-isekai genre).
In general, if they're not going to benefit from being isekai....then just make them normal fantasy. So many good fantasy animes out there that would have been hurt by making the protagonist just an isekai'd character.
The generic ones are just formulaic escapist-fantasy genre-writing that's oversaturated because "what if you could just not be dealing with all the bullshit of this broken, dying world anymore, and also you get a free harem or something idk" is as comforting a slop trough to disaffected young men today as "what if you could be a big strong manly adventurer guy and get exotic damsels in distress as special good boy prizes" was with pulp adventure or sword and sorcery novels in the early-mid 20th century. You could probably toss in other formulaic genre slop like the Noir genre too.
Some of them actually do something interesting with it, though, and I've noticed an increasingly common gag in normal fantasy series of throwing in an isekaid character who's not in the main cast. Like A Wild Last Boss Appeared! is formulaic genre slop, but also actually makes the way the world isn't just a generic fantasy world but specifically a generic fantasy world written as a backdrop for a shitty MMO that's also been heavily influenced by officially recognized fanfics from its players a meaningful point, with the protagonist literally stepping into the shoes of a fictional character he created and wrote but who also had this entire separate existence full of all the little details the game couldn't provide or that would had to be there for the character's story to make sense. It basically ends up as "what if Overlord was actually well written and had good worldbuilding" while still being slop with mistakes and plot holes that the author points out in the volume notes.
I can't believe I'm saying this, but the Alicization seasons are actually the best that mainline SAO ever was. I mean they're still gross and even more full of the other kind of SA than before, but the storyline of literal artificial human souls growing and developing within their own artificial world and how it handled the fact that in the shitty fantasy world some scientists built they used the same artificial literal human souls to power the designated ontologically evil antagonist races (tl;dr: they were also people and had an innate bias towards wanting peace and coexistence instead of eternal apocalyptic warfare, and had to be forced into that role through violence) was actually pretty good. The only genuinely ontologically evil characters in the seasons are American PMCs working for the CIA. And the best part is Kirito is either depowered or in a coma for almost the whole thing.
Still problematic slop though.
.....huh......I quit fairly soon into SAO to be honest; I had no idea the story went there. I think I quit after the second story (where they're in another game and Kirito's friend was being kept hostage) and didn't keep up to date with the rest of the story (which was easy given how long the show continued on for). I may have to go back to SAO to see this storyline.
Yeah, the mid seasons are mediocre at best, with the best of them being the GGO one (and the GGO spinoff is actually sort of good, at least the first season; it was written by a completely different author with a completely different cast; still had some problematic issues to it though). The two most recent ones (the two Alicization seasons) were fun to hate watch and just happened to pleasantly surprise me by doing some things pretty decently and generally being better than the earlier seasons. Still problematic slop though.