That's not true. We've seen multiple conflicting versions of the late 24th and early 25th century, timelines that have clearly been wiped out -- "All Good Things...," "The Visitor," "Timeless," "Shattered," and "Endgame."
And like I've said, lack of contradiction is not proof of consistency. Those various far futures may not explicitly conflict with each other, but they don't explicitly reference each other either. So you're jumping to a conclusion that can't be proven, and that doesn't even make sense in the broader context of a universe where the mutability of time has been explicitly demonstrated almost from the beginning.
No, it really, really doesn't. Admiral Janeway in "Endgame" came back from roughly the same time as adult Alexander in "Firstborn," the first decade of the 25th century. And we've seen four distinct versions of the 2390s: in "Timeless," in "The Visitor" (the portions set in Jake's mid-30s), in "Shattered" (with grown-up Naomi stuck in Astrometrics), and in "All Good Things...". Plus Trek literature has given us yet another alternate version of the same era in Millennium: War of the Prophets.
Ahh, but those aren't necessarily the same future! Remember, at the end of "Future's End," Captain Braxton's own history had been changed so that he had never been thrown back in time and had to live on 20th-century Earth. Yet the Braxton of "Relativity" remembered living on 20th-century Earth and had a grudge against Janeway because of it. They contradict each other, which is really just sloppy writing, but it certainly doesn't support your case that they're the same unalterable future. (The fact that Braxton was played by a different actor doesn't help either, though that doesn't prove anything.)
You're making the mistake of thinking of this as though it were something real instead of what it really is, a story. Since it is only make-believe, the "laws" of time travel are only what the writers need them to be for the sake of the story, and it's important to keep that in mind. As I've already explained, there are very good story reasons why Trek writers have always made a point of portraying the future as a mutable thing, so that they won't unduly restrict the storytelling options of future writers, themselves included. Your assumption just doesn't fit with either the in-story evidence or real-world common sense.