I mean if you're a royalist it will be a big moment in history. For most it's just another famous person dying (admittedly she ranks quite high in terms of fame).
The same way Michael Jackson fans consider his death a big moment in history.
Atleast with the latter you can still experience his legacy through his music.
Again, how big it is relative but most people, more than Michael Jackson, will remember this death
I feel you are looking at this from a very simplistic view (not saying you are simple or dumb etc., please don't take it that way). Realising that this is a historic moment (even if for a brief moment) doesn't mean people think it will impact their lives more than 1 day. It's just a 'wow she's the only monarch I've known so it's weird' moment. She was widely popular too so has more attention (even if we disagree that she should be this popular)
Also for many anti monarchy people, Queen was still well liked (relatively as a person as she came across as a sweet old lady.... even if she wasn't) - it may be the beginning of the end for the Royals cos nobody really likes Charles. For that reason, it could even be more historic