One should investigate closely the connection between (well-supported) theories of physics and "reality", which, I gather, in this case means something like "the ontology of the universe".

Is the universe "actually made of" a flexible substance that moves according to the equations of GR? In one sense, it doesn't matter. It acts like it does, and so well that we can make very precise predictions about what will happen.

Suppose a naysayer in 2010 said "Well, GR is a nice mathematical formalism, but mathematical formalism isn't reality. It's preposterous to think space is actually made of this mysterious flexible substance. You'll never see gravitational waves. It's a fiction of the math."

The naysayer has conflated the claim "GR is ontologically true" with the claim "GR makes accurate predictions." The first is irrelevant, and may be freely denied without casting meaningful doubt on the second, which has been well-tested for a century. It would be a great surprise to conduct an experiment and learn that GR mispredicted the result.

QM predicts QC. To doubt QC is to doubt that QM accurately predicts experiments we can conduct. In this case, once again, a century of experiments cuts the other direction. The failure of QC would be the surprise, not its success.