When does the colloquialism “Rocket Science” become obsolete?

Maybe when spaceships will not be based on rocket engines. That said, at the moment it seems difficult to see what could do better than a ionic engine, given the huge margin of progress there exist in this area.

Also the main problem is not a new propulsion technology, but a new energy source, passive like in solar sails or active like in liquid/radio-active rockets.

After thinking about it, the origin of this expression may be the difficulties there was to understand how the combustion chamber works. Actually they had to create instabilities to obtain a stable exhaust jet.