I mean, Blizzard’s multiple universe system is convoluted and nonsensical, yes, but that’s besides the point.
They can say it’s decaying all they want, but it doesn’t give a timescale. Will it take five years? Ten? Five hundred? The fact that it’s unstable doesn’t give it any kind of urgency in the plot, because it’s going to last exactly as long as they need it to before they pull it out for a new expansion/patch again.