While I'm Still trying to sort out how I feel about the movie, and a whole lot of it isn't positive, I think I can explain some of the trepidations some fans (old or young) have about it. With a tastey metaphor!
I get what you're saying, but I think one point of disconnect is that, for most fans, this moment came long ago, and we've had years to adjust to it. For many it came with TNG, Star Trek without Kirk, Spock, and McCoy - even many people who eventually grew to like TNG had that initial "WTF? That's not Trek!" reaction. For some it was DS9, Star Trek without a starship. For some it was the blandness of TMP, or the militarism of TWOK, or the anachronisms in ENT, or Kirk's death in Generations, or the general awfulness of the last two TNG movies. For me personally, it was Voyager, the first Trek series I really didn't like - I'd been watching Star Trek since before I could remember, thanks to my aforementioned mother, and after three series and seven movies, it was a bit of a shock to the system that all of a sudden Star Trek sucked.
Trek has experienced many "endings" as well - whether it was the barren desert of the 70s, or the differences in the movies, or TUC, or Kirk's death in Generations, or the shift to TNG-only movies, most of us TOS fans realized that TOS was over. After INS failed to impress, and the comeback effort was the limp noodle of Nemesis, most of us TNG fans realized that TNG was over. Most DS9 and VOY fans knew those series were never jumping ot the big screen. After INS, NEM, and ENT, Trek as a whole was as dead as it's been since the 70s.
Most of us have adjusted to these changes. Some people ignore the new as not "real" Trek like the old stuff they prefer. Most of us simply expanded our definition of "real" Trek. The definition of Trek expanded to include stories about getting older instead of stories about young derring-do, to include stories without the original crew, to include stories not based on a ship, to include things that are mutually inconsistant and yet still both Trek. Now Trek has expanded to include stories based in alternate timelines. This may be the biggest fundamental change in Trek since the premiere of TNG; it's understandable that some people have a problem adjusting to it.
But whether it's grape or lemon-lime, it's still mostly water and high fructose corn syrup, it's still sweet and fizzy and cold and refreshing.