There is no doubt that quantum mechanics has seized hold of a beautiful element of truth and that it will be a touchstone for a future theoretical basis in that it must be deducible as a limiting case from that basis, just as electrostatics is deducible from the Maxwell equations of the electromagnetic field or as thermodynamics is deducible from statistical mechanics.To understand some of the reasons Einstein felt like this take a look at the Measurement in quantum mechanics FAQ
I do not believe that quantum mechanics will be the starting point in the search for this basis, just as one cannot arrive at the foundations of mechanics from thermodynamics or statistical mechanics. - Journal of the Franklin Institute, V 221, p 313, 1936, quoted from: Abraham Pais, Subtle is the Lord, p 461, Oxford University Press, 1982.
I consider it quite possible that physics cannot be based on the field concept, i. e., on continuous structures. In that case nothing remains of my entire castle in the air gravitation theory included, [and of] the rest of modern physics. - Einstein in a 1954 letter to Besso, quoted from: Subtle is the Lord, Abraham Pais, page 467.To lean more about how such a theory might be developed take a look at ``Developing a local deterministic theory to account for quantum mechanical effects''.
Modern physicists have a wide range of views on these issues.
While the mathematical developments which constitute quantum mechanics have been outstandingly successful in describing all manner of observed properties of matter, it is fair to say that the conceptual basis of the theory is still somewhat obscure. I myself do not properly understand what it is that quantum theory tells us about the nature of the physical world, and by this I mean to imply that I do not think anybody else understands it either, though there are respectable scientists who write with confidence on the subject. - Ian D. Lawrie from a A Unified Grand Tour of Theoretical Physics p. 95.Most are complacent. Many revel in the thought that modern physics has transcended classical ideas. The recent books by Penrose are a good example. This thought is far more flattering to the ego than to believe something is fundamentally flawed in ones understanding and to have no idea how to address the problem. The intellectual modesty and honesty of physicists like Lawrie is far more likely to lead to new physics than the arrogance of those who would dismiss the conceptual problems in physics.
Let us consider two measuring apparata located in two different places A and B. There is a knob a on apparatus A and a knob b on apparatus B. Since A and B are separated in space, it is natural to think what will happen at A is independent of the setting of knob b and vice versa. The principles of relativity seem to impose this point of view if the time at which the knobs are set and the time of the measurements are so close that, in the time laps, no light signal can travel from A to B and vice versa. Then, no signal can inform a measurement apparatus of what the knob setting on the other is. However, there are cases in which the predictions of quantum theory make that independence assumption impossible. If quantum theory is true, there are cases in which the results of the measurements A will depend on the setting of the knob b and/or the results of the measurements in B will depend on the setting of the knob a. - "Bell's Theorem without Hidden Variables", P. H. Eberhard, Il Nuovo Cimento, V38 B 1, p 75, Mar 1977.Had Einstein been alive at the time of Bell's result the history of physics in the later half of the twentieth century might have taken a different turn. I think Einstein would have seen this as the experimental key to prove the correctness of his position. He would have considered experiments to test this to be of the highest importance and his opinion would have had considerable impact on the priorities of many experimentalists. While recent experiments have producing compelling results they have failed to simultaneously close the two major loopholes.