What we want for ourselves and our descendants is obviously no (or at least no longer) the same a reproductive fitness.

I don't think any non-human animal ever (consciously) wanted reproductive fitness as such, this being a long-term, multi-generational desire that humans may be the only animal capable of caring about, and I don't think all that many humans do. On second thought, maybe they do, in the sense that many humans want children and grandchildren. But the things we typically do want by and large enhance our reproductive fitness, so reproductive fitness is, in practice, a good proxy for what we want (though it might be more accurate to say that what we want is a good proxy for reproductive fitness). Good music, for example, comes from good musicians, and a high musical ability is thought to be closely related with sexual selection. Same with athletic ability, intelligence, empathy reliability, trustworthiness, good parenting, and so on and so forth. Pretty much everything we rejoice in about ourselves, Is closely tied one way or another with reproductive fitness.

no reason to rejoice about a future in which humanity is made up of crack-heads.

When I survey the traits that have been and currently still are correlated in the human species with greater reproductive success, I find admirable traits such as those I've listed above. You are speculating that at some future date, all these other traits will be swamped by a tendency of welfare crack-heads not to use contraceptives. I don't have any proof that your scenario is improbable, but still I think it is.