"By the book"...
As a prod the system only gets the Referee's imagination going. There's nothing either Old School or "by the book" about being stuck with a WPP you don't like. That's simply masochism.
Different Referees will have different points where their imagination stops having fun or "loses briliance" making an explanation of a given UPP, but there's nothing to suggest in the text that the Referee is stuck with such a WPP. I tend to keep going with a WPP until I sort it out... but I don't assume everyone wants, willl, or should do this.
As for focusing allowing for multiple worlds, Book 3 says...
So, again, it is up the Referee to decide how far he or she wants to to in creating environments. There are no dictates in the Classis Traveller rules. Only starting points for the Refree and Players to begin with and expand from based on their interests and fun.