It's not something you rush out at the last minute, but it might be something you plan a long time ahead as "our interesting stuff might not work out, so let's do a huge redesign too to be confident we can pretend to be releasing something excitingly new either way".

(I don't particularly have an opinion that this was their line of thinking, just pointing out that for a company like Apple they would have been thinking "what if X isn't ready in time" months or even years before the point of actually knowing if X is it isn't ready on time.)