BG1 & BG2 used the current 2e rules at the time. That's why people want BG3 to use the current 5e rules. It also helps that 5e is way more popular by orders of magnitude than all previous editions combined.
As for your tonal comparisons, think of it like a comic book. If you pick up an issue of Batman, you expect it to look, feel, and read like a Batman book no matter who wrote it and who the artist is. The franchise has a set tone. It's not unreasonable for people who have been playing BG games for 20 years (and to a lesser extent NWN for 3e) to expect a consistency in the tone and presentation of the brand.
Good comparison with the Batman comics, there have been many adaptations of Batman and visual or tonal changes made to his character and world. Some were original hated by the fan base, while at the same time creating a new fanbase.
Also I think that saying people have been playing BG for 20 years is an over statement. How many times in the last 20 years have you played BG, how many times has it become relevant again to new players. It has been around for 20 years but not necessarily continuously played/replayed. I have many games that I cherish from 20+ years ago, like Ultima 7 Serpent's Isle. I have played and beat that game one time in my life but it was so impactful I still remember the story and it filters my opinion about games and story to this day.
Do I think games should use the same graphical style, user interface, non-rotating isometric view, real time combat and user input typing to chat to NPCs to this day? No, we have evolved gaming interfaces and created new ways of telling a story in the video game medium.
Back to 2e rules, it has been stated many times, but even BG1&2 deviated from the 2e rules to make a better translation into a video game.
