Einstein did admit that his false constant-speed-of-light postulate killed physics. Here is the story:

Einstein could have based his theory on the discontinuous particle concept of light which presupposes variable (dependent on the speed of the source) speed of light. Instead, he based it on the continuous field concept by "borrowing" the constant (independent on the speed of the source) speed of light from the ether theory:

Banesh Hoffmann: "And if we take light to consist of particles and assume that these particles obey Newton's laws, they will conform to Newtonian relativity and thus automatically account for the null result of the Michelson-Morley experiment without recourse to contracting lengths, local time, or Lorentz transformations. Yet, as we have seen, Einstein resisted the temptation to account for the null result in terms of particles of light and simple, familiar Newtonian ideas, and introduced as his second postulate something that was more or less obvious when thought of in terms of waves in an ether. If it was so obvious, though, why did he need to state it as a principle? Because, having taken from the idea of light waves in the ether the one aspect that he needed, he declared early in his paper, to quote his own words, that "the introduction of a 'luminiferous ether' will prove to be superfluous."
Relativity and Its Roots, p.92 https://www.amazon.com/Relativity-It.../dp/0486406768

Albert Einstein: "...I introduced the principle of the constancy of the velocity of light, which I borrowed from H. A. Lorentz's theory of the stationary luminiferous ether..." https://en.wikipedia.org/wiki/Lorentz_ether_theory

"And then, in June, Einstein completes special relativity, which adds a twist to the story: Einstein's March paper treated light as particles, but special relativity sees light as a continuous field of waves." http://www.pbs..org/wgbh/nova/einstein/genius/

"The two first articles (January and March) establish clearly a discontinuous structure of matter and light. The standard look of Einstein's SR is, on the contrary, essentially based on the continuous conception of the field." http://arxiv.org/ftp/physics/papers/0101/0101109.pdf

In 1954 Einstein realized that, by wrongly assuming that the speed of light is independent of the speed of the source (that is, by basing his theory of the field concept), he had actually killed physics:

Albert Einstein (1954): "I consider it entirely possible that physics cannot be based upon the field concept, that is on continuous structures. Then nothing will remain of my whole castle in the air, including the theory of gravitation, but also nothing of the rest of contemporary physics." John Stachel, Einstein from 'B' to 'Z', p. 151 https://www.amazon.com/Einstein-B-Z-.../dp/0817641432

Judging from the quotation below and his cursory lecturing on relativity, Feynman knew how fatal it had been for physics to be "based upon the field concept":

Richard Feynman: "I want to emphasize that light comes in this form - particles. It is very important to know that light behaves like particles, especially for those of you who have gone to school, where you probably learned something about light behaving like waves. I'm telling you the way it does behave - like particles. You might say that it's just the photomultiplier that detects light as particles, but no, every instrument that has been designed to be sensitive enough to detect weak light has always ended up discovering the same thing: light is made of particles." QED: The Strange Theory of Light and Matter p. 15
https://www.amazon.com/QED-Strange-T.../dp/0691024170

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