This article seem to point a key difference: old trek = episodic (boring), new trek = serialized (exciting). Yet the two most appreciated series (to my impression) are SNW and LD, both being episodic and highly faithful to the "old" series, while refreshing some themes and presentation, in a positive and modern way.
Discovery is not hated because it's serialized, or "gritty" (DS9 is gritty, with genocide and war crimes, DIS is cringey, with heavy and dark themes carried out as a 12 years old would). DIS is hated because of poor writing, low stakes, monolithic, brainless and spineless characters and a general lack of respect for established canon that can only be resolved by (SPOILER BELOW)
Yeeting them out of our time and literally erasing them from history. And somehow even in the far future their incredible technology is advanced.