The view that Dirac is putting forth, that the fundamental equations need to be simple, is certainly not the modern one endorsed by theorists today. Since the 7

The view that Dirac is putting forth, that the fundamental equations need to be simple, is certainly not the modern one endorsed by theorists today. Since the 70s or 80s, the point of view that has come to dominate is that our most accurate theories (quantum field theories) are what are known as "effective field theories", where the adjective 'effective' refers to the idea that they are only only low-energy approximations to some other more complete theory. In this point of view the complete theory would have more complicated, even non-renormalizable terms that violate conservation laws, but these additional terms are suppressed by powers of something like the plank scale and so are virtually undetectable at low energies. Thus the low energy approximations (the effective field theories) have the simple, renormalizable form that we know. There is a good accessible discussion of this in Weinberg's popular science book 'Dreams of a Final Theory'.