However, the paper was trying to model allele mutation rates using a particular function for distribution of mutation fitness, not describe a complete model of evolution. He deliberately skips important processes. From the end:
"Whether such a small rate
of deterioration in fitness constitutes a threat to the survival and
welfare of the species (not to the individual) is a moot point, but.
this will easily be taken care of by adaptive gene substitutions
that must occur from time to time (say once every few hundred
Here is a paper which describes this process:
Selection and mutation have many levers to pull.
Edit: I see now from reading that Kimura's ideas are still controversial.