>For example, the Theory of Relativity smashed Newton's Law of Gravitation that had been fully consistent and useful for quite a while, even though it was wrong.
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>Perhaps that is what people mean when they observe that something is "only" a theory? Consistent/accepted today, discredited tomorrow?

Newton's theories were not completely wrong, nor are they completely discredited. They are an important basis for science, up to this day.

Some parts of his world-view - or, in general, of what is known today as "classical physics" - was simply found to be not entirely accurate under certain extreme conditions, especially: (a) at speeds which are significant, compared to the speed of light, and (b) at very small, i.e. sub-atomic, scales.

For example, if one car goes at 50 km./h. to the north, and another one goes at 50 km./h. to the south, their relative velocities (according to classical physics) are 50+50=100 km./h. If you use the corresponding formula of the Special Theory of Relativity, you will get a different value. But the correction is too insignificant to be of any practical value.

Classical physics is still taught in schools, and it will continue to be taught for the foreseeable future.