The only ones calling the shows "alternate timelines" are fans uncomfortable with change.
I think it's very much a motivation and goal of these shows. Maybe not to the extent some fans want though. They are after all making a show for and in the 2020s. And fiction will always be mutable, especially in along running franchise.
Apart from the makers of the 2009 movie who even called that new era a different timeline? Abrams didn't say he was a big fan of Trek either... But a new timeline can be both a curse and a blessing; the latter being you have a clean slate to do anything with regarding the same recognized and established character names and backgrounds...
Another example comes to mind -- Look at Shakespeare's plays. Language verbiage structure has changed since the 1500s and other arguments aside, nobody speaketh liketh thateth anymore (though it was cool when Q tried iteth). The same plays were recycled but played by different actors since the actors from 1593 were more or less dead a few centuries later, but nobody's perfect. Not to mention that plots would be tinkered with to be more "modern", whatever that can mean...
Granted, Shakespeare's profession wasn't as
large or varied expansive when compared to the 20th and 21st centuries either.
And it was Shakespeare who also noted the limited number of tropes and combinations, the real issue is using them in a new way that makes them feel fresh again...?
Not to mention, most of the "classic movies/shows" were derided at the time when they were first made for being:
- too different
- too weird
- too expensive to make
- too reminiscent of something else (except they all borrow at some point)
- etc
and/or any combination of those.
The Wizard of Oz is one example of a movie that was a flop that was well-regarded later... (think "cult classic" except TWoZ had far more fans over time compared to other movies that became cult classics over time...) which had some remakes, of which many were rubbish because the same spirit was there and/or the fans were full of themselves or anything in between.
Battlestar Galactica from 1978 was a retelling of parables, so was 1970s Doctor Who... how well they did it is arguably subjective as well...
So, time will tell... it's still an honor when some franchise is continued or remade or rebooted - it lives on, in a sense. Can't say the same things about awesome shows such as "My Mother the Car", "Captain Nice", and others that are too cringe to bear...