The old stuff is still there but it allows something new and fresh to be created.
I think what ChristopherPike
objects to is the "original plan" in which the old stuff wouldn't be there, because Earth (apparently) would have been destroyed alongside Romulus, and many characters would have been killed off. Setting the new films in a new timeline is a smart idea, in my opinion, and for exactly the reasons you outline above, but causing widespread destruction in the old timeline before you leave seems to defeat the entire elegant purpose of the move, and would annoy a lot of people.
It took me a while to accept the loss of Romulus, for instance; I'd still be grumbling if anything else had been destroyed alongside it