Many times Star Trek has taken us to the future only to reset the status quo at the end of the story arc. Tapestry (but in reverse?), that time Voyager crashed in the ice, and all that.
How likely is it that Discovery went to a mutable future, just one of many, especially with the Temporal Cold War, Carl, Q, Trelane, Janeway, the HMS Bounty, and any number of other temporally active agents out there in time? How locked in is the 32nd Century?
Voyager Before and After pretty specifically tells us that whenever someone jumps in time, the fact that they have done so, and the actions that they take, affect the timeline. Any time time travel is involved, we are seeing only one possible timeline, not necessarily the timeline.
Of course, it’s easier to use that reasoning in an episode like Before and After, because the time travel involved is backwards, but I think it’s reasonable to assume it’s true the other way, too. After all, the Kelvinverse isn’t identical to the Prime timeline even before the Narada arrived.
But removing Discovery from the timeline seems to be consistent with the prime timeline post-Discovery season 2 (in TOS etc) - e.g. Spock not talking about his human adopted sister, no further use of spore drives, and so on. It’s certainly explicitly the timeline of SNW (which makes multiple references to the events of Discovery s2) and therefore the timeline of Lower Decks.
That suggests the prime timeline as we know it is an altered timeline caused by Discovery’s jump to the future.
Kelvinverse was different from the Prime timeline before the Narada arrived because people past that point might not travel backwards in time the same way they would have without intervention.
I just happened to be watching that episode of Voyager when I came across this post, so it was currently front-of-mind for me.