Except it happened the other way around: E=mc^2 was 1905, people only had the means to test for that mass difference after the discovery of the neutron in 1932.
They hypothesised earlier than that, of course, but the experiments followed updated laws nearly 30 years after the new laws had become necessary because of lots of previous experiments showing Newtonian physics was wrong.
And sure, GR will be replaced with something else (it predicts but cannot fully describe black hole interiors), but…
…but expecting that upgrade, whatever it might be, to have the ability to defy known conservation laws in a measurable way in a device made of quarks and electrons, is akin to thinking one possible refinement to the shape of the Earth (curved, sphere, oblate spheroid, …) is the existence of a 29 mile long tunnel connecting Hawaii to Moscow.
Any new physics will almost certainly involve things we can only just build at this point, not stuff like the EM drive which we could’ve built by accident from WW2 onwards.
