Science runs out of things to discover all the time. The last big point when everybody thought that now "everything is understood" was in the middle of the 19th century. Mathematics was developed enough to descibe that classical non-relativistic point-mechanics world well enough. Everything seemed fine.
Then came relativity and quantum mechanics, and in the wave understanding all these phenomena there was an time when the theories could be used verified (with the "low hanging fruit" first). Fact is, building technology from a theory is the last step in using the theory. This step happens now for QM with the coherent of single quantum controls. Only when you have such technology, you can actually test the limits. I should remind here that, at the time of implementation the Michelson-Morely experiment was absolutely state of the art, using all understanding in the design and the newest technology.
Now we are at a similar point. We need extremly complicate technology (Quantum Computer, Gravitational wave detectors) to bring the experiments to the limits. Absolutely nobody nowadays can tell if QC will work (i am a former QC researcher). If the interpolation between the "macroscopic/collective" Quantumphenomena and the few quantum entangled systems indeed will exist is something which we expect, but it is an *unproven Hypothesis*, a test of a theory in limit which was never tested before.
Maybe everything works out according to theory, but possibly not. The same is true for Gravitation.