I think you're really overthinking this. My point is the opposite; they should announce new features (of course) but they should (as they did with teleport) announce where the 1st chance is so people can plan accordingly. (or start putting out updates to spire the day spire starts, but I can see that being impractical)
And I think you're under thinking it.
They told us when they were changing the prizes (Monday) and what they were doing, they didn't have to do that at all. It only affects people who play the spire, and only those who get all the way to the top of at least one stage, and only if they get all the way to the top of at least on stage on the one particular day, yesterday, and only if they rolled a number from x to x+10 /100. It isn't a new thing, magic buildings have been in the game all along.
It isn't even new for the Spire, the original implementation on Beta induced magic buildings as prizes but they had to yank them because it broke the game. All it is is putting back a feature from before.
Meanwhile the risk: if they specify a time and can't meet it, people will get mad at them and threaten to rage quit. If they add them and then it turns out that (like last time they added magic buildings as prizes), it causes problems because the buildings are different for Elves vs Humans and they have to roll it back, they get people upset and threatening to rage quit.
Worrying about it is pretty much a text-book definition of 1st world problems.