Mission Creep is when the mission changes to something new. That is a bad thing.
But people use that term whenever a government program expands.
Often missions start small and grow big. That is because 1) The scope of the problem was not realized when the program started.
2) The program's scope was realized and they correctly decided to start small (which they may or may not have informed everyone ) and make sure they got it right before then went big. Often people doing this intentionally do not mention this to their enemies - as the enemies will use it as an excuse to not do the work. Why should we save the entire environment? Why should we get rid of ALL of polio? Those things cost too much!!!
The worse case is when the people against the original program complain about 'mission creep' when the plan to save the few people who lived to 65 from abject poverty suddenly becomes saving all the many many people that live to 65.
This is not mission creep. This is simply maintaining the original program and making sure it worked, even though the problem is now a lot bigger.