Sarcasm aside, Dr Adequate is right. You haven't come close to showing that there must be an inevitable decline.
The pocket mouse population has gained multiple alleles for dark fur. This is an increase in genetic diversity. If this increase was wiped out, such that only the dark mice were left there would still be no overall decrease in diversity, from the state before the mutations occurred but there would be a change in phenotype.
Thus it is obviously false to say that evolution requires an overall decrease in genetic diversity.
And that has been obvious all along to everyone who has actually thought about the issue.