When do we have to give up on an old theory and try to come up with a new theory?

Question

Would it be when the old theory under goes changes?

Also, how would I compose my answer to this question into a paragraph?

This happens when we suspect that the theory is not perfect. E.g., the Standard Model of particle physics has not been falsified experimentally (except for the fact that neutrinos are not massless, but that can be repaired), but we know that it cannot describe Nature correctly, because it doesn't include gravity (and it cannot be changed to include it in a trivial way).

Also there are are some other technical problems with this theory, but so far no experimental violations. But this is enough for physicists to search for new theories.

If, on the other hand, there are no good reasons to believe that a theory is false, then you won't see many physicists searching for alternative theories. A good example is quantum mechanics. It is believed by almost all physicists that the principles of quantum mechanics are exactly true. There are only a handful of physicists who work on alternatives to quantum mechanics.

Thanks!