sigh
The fact that you even have to ask means you weren't really paying attention.
No, I think the fact that I very much have to ask means that I'm the one paying close attention here. You're making a lot of very telling assumptions.
The DM creates a world, they refuse to let any player do anything to change their world before session 1. Following?
Kosher so far.
Okay, now during the campaign, the players build the Barony of Bob. This is a new thing that they added to the DMs campaign, the thing the DM previously would never have allowed.
It's a thing high-level PCs do all the time.
Then, after that campaign they are starting a new campaign and find out that the Barony of Bob was destroyed utterly by Mel, everything the PCs built and added to the campaign world is erased, and the world is.... back to the status quo the DM set before the last campaign, the thing that they would not allow the players to change.
I never mentioned anything about one campaign ending and another beginning. Or that our Chaotic 15th level magic-user in question was necessarily an NPC under the DM's control. He
could have been (since, in a sandbox game, it's incumbent upon the DM to keep track of
all the high-level NPCs' plots, comings, and goings, precisely because the PCs aren't privileged over the NPCs—this campaign style largely treats them equally), but then again, he could just as easily have been a player character operating at the same time as Sir Bob the 10th level fighter. Maybe a former party member of Bob's before Bob moved on from dungeon-delving and switched to domain-building; maybe a former member of an entirely different party operating in the same campaign. (Are you working under the assumption here that "a campaign" means that the action only follows one party of tight-knit player characters, all staunch allies and bosom friends, and that only one mostly-stable PC party can be at work in the setting at a time?)
Or, let us say that the build the Barony of Bob, and then at the start of the new campaign, the DM sets the Barony to be the villain of the new campaign, with the player's goal being to destroy the Barony. Which... puts the world back into the status quo that the DM didn't want the players changing.
So there's another set of assumptions here (and in the previous paragraph), that the destruction of Bob's castle must somehow involve villainy on either one side or the other, and that said villainy must have been orchestrated by the DM. And that the DM is "setting" the players' goals. (That one is
very odd to me.) And that the DM's goal in doing this is restoring the world's status quo, which is just… telling, again, but not accurate regardless. Because the initial status quo was empty wilderness. If Player #1 builds a barony there, that's the new status quo. And then, if Player #2 (or NPC #743) comes along and destroys that castle, now there's a smoking magical crater there, which is hardly the original status quo. (If the barony had peasants and citizens, it's also an open question as to whether they're all dead now, or conquered by the interloping magic-user, or whatever else. We're getting further and further away from that pristine wilderness!)
To me? This is clearly a sign that the DM being "perfectly willing" to allow the players to change their gameworld through the campaign is a lie and a smokescreen. Because the reality is that the DM is just going to find a way to destroy their contributions and reset the world to how the DM wants it to be.
You're bringing the reset button to this example. (And notably ignoring the other example I gave, of a PC cleric who literally overthrows a DM-created kingdom.) It's all you. It's another curious assumption you're making about this hypothetical DM's motivations.
And that is why it matters. Because if the DM is just going to hand-wave and destroy the things the players made, then have they really allowed the players to affect the world in a meaningful way, or do they just want the status quo of their vision to remain?
All you, buddy. You can posit a DM reset button all you like, and you can draw whatever speculations and accusations from it that you like, but at that point you're just talking to yourself.
I gave some fairly straightforward examples of PC actions that could alter the face of a campaign setting. You're inexplicably fixated on the notion that something a PC
does can also be
undone (quite possibly by another PC).