Sure you can chalk up the had writing to generic fantasy 101 or whatever. But is that acceptable in a game held in such high regard? That's a big reason the game is an 8/10 for me and not this masterpiece many claim. Games like deux ex, kotor 1/2, mass effect 1, fallout 1/2, planetscape the witcher and more have really good writing and companions/npcs/villains that put this game to shame. You could say bg3 has better gameplay but that's like half the score. The other have is the writing, characters and story. When it is lacking in half of what makes a great game, I don't understand all the praise.
I dunno, of the ones you mentioned I think I prefer BG3's writing to The Witcher (which I can't stand), Mass Effect 1 and KOTOR 1 (BioWare's writing bores the shit out of me), and Fallout 2 (funny when you're 12, less so 25 years later, and does a lot of damage to the Fallout setting).
BG3's writing isn't even bad, it's just Saturday morning cartoon stuff, a bunch of larger-than-life heroes out on an adventure where most things are rather black and white - so, basically, Dungeons & Dragons. The plot is a thin excuse to be shuttled through all these exotic locations, meeting bizarre characters and ending up in surreal situations. It sort of felt like the scant few good, high-concept parts of Neverwinter Nights (like the snowglobe world) with all the absolutely worthless chaff removed. The writing only really goes to pieces in the third act, and even that has a few standout areas like the House of Hope which I found pretty evocative and unnerving.
Agree that the companions are a mixed bag; La'zel and Minthara are generally very good, Gale is alright albeit with a stupid backstory, Shadowheart is fine if uninteresting, Jaheira and Minsc feel appropriately familiar. I see why some people hate Karlach but I thought she came across as refreshingly amiable and normal in a cast that's otherwise a bit over the top. I liked Astarion at first but was a bit sick of him by the end. Halsin and Wyll are very boring but I don't know if I'd accuse either of being badly written as such.