See above. But again I would point out that even if it were true that new alleles are added by mutation, consider this: a mutation CHANGES an existing allele, it doesn't add something newer than a different expression of that particular gene.
Please review a description of the different types of genetic mutations that there are:
Mutation: Classification of mutation types.
One type of mutation is allele duplication, in which a copy of a gene is inserted, such that where only one allele existed before there are now two. Consider the gene for skin color. If only one such allele existed then we would expect to be either black or white, but since multiple alleles for skin color exist we instead have a rich range of different shades of brown. This is an increase in diversity.
Then there are mutations that can change the function of a gene, for example alpha-lactalbumin, part of milk production, originated from lysozyme, an anti-bacterial protein. So when the gene for lysozyme changed to one for alpha-lactalbumin, it could no longer produce lysozyme. However, since the gene for lysozyme had been duplicated, mammals' ability to produce that protein was not lost.
It is by the duplication of genes to create multiple alleles that that trait is preserved as some of those genes then either lost functionality or gain new functionality.
You're only looking at what decreases genetic diversity and not at what increases it. Take the analogy of your household budget. You keep paying money out for bills, groceries, and other expenses. If that keeps up, you will end up with absolutely no money at all! Well, that is what you must conclude as long as you ignore the money coming in. Income balances against expenses; they both play a role that must not be ignored.
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