I simply have to ask: so what? I place no particular terminal value on evolution itself. I see nothing wrong, neither aesthetically nor morally, with simply overriding evolution through human deeds, the better to create the kind of world that, indeed, we living humans most want to live in.

I don't think I was clear enough. I'm not stating that it is value-wrong to alter the environment; indeed, that's what values push people to do. I'm saying that while the direct effect is positive, the indirect effects can be negative. For example, we might want casual sex to be socially accepted because casual sex is fun, and then discover that this means unpleasant viruses infect a larger proportion of the population, and if they're suitable lethal the survivors will, by selection if not experience, be those who are less accepting of casual sex. Or we might want to avoid a crash now and so transfer wealth from good predictors to poor predictors, and then discover that this has weakened the incentive to predict well, leading to worse predictions overall and more crashes. Both of those are mostly cultural examples, and I suspect the genetic examples will suggest themselves.

That is, one of the ways that values drift is the environmental change brought on by the previous period's exertion of their morals may lead to the destruction of those morals in the next period. If you care about value preservation, this is one of the forces changing values that needs to be counteracted or controlled.