If you intend to travel in more than one country in the Schengen area, you must apply for a Schengen visa at the mission of the Schengen country where you intend to stay longest. If you intend to stay in each Schengen country for the same length of time, you must apply for a Schengen visa at the mission of the first Schengen country you intend to visit.
(Emphasis mine)
This is about the application process, and about intentions. Circumstances change, and what was intended at the time of application may not necessarily happen.
There was a similar thread here some years ago where the conclusion that was reached (based on personal experience of a number of those who posted) was that the powers-that-be accept that plans can change, and that there is no need to apply for a new visa if a trip to Country A gets cancelled and the visa holder subsequently needs to travel to Country B during the validity period of the visa, as long as the visa wasn't obtained under false pretences in the first place.
Of course interpretations may have changed since... which would be a shame, because it would undermine much of what Schengen is meant to be all about.