Well, that is a half-truth. The same can be said about general relativity: both QM and GR can be considered borderline cases of a hypothetical, unknown theory of everything, QM being the "small borderline" and GR the "big borderline".

Saying that GR is "more valid" than QM is limited to the observed systems and not a general physical statement.

No, because quantum mechanics and general relativity don't have anything to with each other. Quantum mechanics is the framework-not a specific theory!-for describing quantum fluctuations, whatever the details of the physical system; general relativity is the specific theory for describing the properties of classical matter in classical spacetime together. It is the unique theory defined by the properties of invariance under diffeomorphisms of spacetime, second order equations of motion and that the spacetime geometry is determined only by the metric tensor.

General relativity, by construction, doesn't address the properties of quantum systems; it's the fact that matter does have quantum properties that imposes developing the framework for describing what happens to spacetime, when the qantum properties of matter can't be ignored, nor their effects on spacetime. This is relevant for understanding the early universe and for resolving the spacetime singularities, that are an inevitable property of general relativity. How to do so is not yet known.

Now it's an additional assumption that, upon trying to describe the early universe and/or how quantum matter can resolve the singularities of classical gravity, it suffices to find the appropriate framework for gravity, so-called quantum gravity, not modify the framework of quantum mechanics.

But this doesn't have anything to do with any choice between GR and QM; there isn't any way in which choosing one over the other, when both must be taken into account, makes any sense.

General relativity deals with spacetime, quantum mechanics deals with phase space; the two are distinct. Time in quantum mechanics-just like in the Hamiltonian formalism of classical mechanics-doesn't have anything to do with the time of spacetime.

You may possibly be interested in a third option, the electromagnetic mechanics umbrella:

Article Electromagnetic and Kinematic Mechanics Synchronized in thei...

That’s an insightful and essential question—one that touches the heart of the 21st-century frontier in theoretical physics. It is true that General Relativity (GR) provides a geometrically elegant, deterministic, and deeply intuitive framework for gravitation and spacetime structure. Einstein’s conception of time as malleable, relational, and embedded in the fabric of spacetime has reshaped our understanding of the cosmos.

However, the assertion that a “better physics integration” must be pursued under a GR umbrella rather than a quantum mechanical (QM) one overlooks some critical facts:

1. Quantum mechanics is not merely a micro-level tool. It is a fundamentally general framework that has survived every experimental test thrown at it, including those involving macroscopic quantum coherence (e.g., superconductivity, BECs) and even cosmological implications like quantum fluctuations during inflation.

2. GR lacks unification capacity on its own. GR is a classical field theory. It breaks down at singularities (black hole cores, the Big Bang) where quantum effects become non-negligible. Unlike QM, it does not possess a mechanism for integrating the other three fundamental forces (electromagnetism, weak, and strong nuclear).

3. Quantum theory contains a richer conceptual structure. When refined (e.g., through approaches like decoherence, relational QM, or QBism), it offers a more flexible and general epistemic framework. It may not describe spacetime as GR does—but it describes observables, correlations, and information dynamics more fundamentally than classical geometry can.

4. The future likely lies in their synthesis, not hierarchy. This is precisely the spirit of modern research into quantum gravity. Whether via loop quantum gravity (quantizing spacetime) or string theory (embedding GR into a quantum field framework), the vision is not of one supplanting the other, but of discovering a deeper theory from which both GR and QM emerge as effective limits.

5. From the quantum measurement side: As argued in my recent comprehensive review [title omitted], the quantum measurement problem itself reflects how our classical intuitions (including those of GR) fail when confronted with non-classical dynamics. Thus, resolving foundational issues in QM might actually be the prerequisite to building a unified theory.

So while GR is grand and geometrically beautiful, the quantum mechanical framework appears to be the deeper language of nature—at least until a future theory transcends both.