It's not stupid to be "out-bred": you're enjoying your own personal life more by keeping more resources for yourself, rather than sacrificing your time and resources on kids with the goal of continuing your family line. Your kids, grandkids, and descendants aren't likely to be of much help after you're dead.
This is an individual vs. society issue: having few kids is better at the individual level, but for society it's not sustainable unless we can figure out how to significantly extend lifespans (i.e., reduce the death rate in-line with the reduction of the birthrate).
One may argue that if you expand "for society" to a worldwide "for humankind", it's perfectly sustainable not to have kids at the current population level.
I think it's a safe assumption that those with religious dogma and those in regions of high child mortality will continue to have many kids, one way or another. The "educated" non-religious class is probably the only one that has a real choice to make. If they keep up reproducing at 2.1x replacement level while everyone else exceeds that level, the worldwide population is just going to continue to grow and then we're in real trouble.
I'm perfectly happy to let "my society" be taken over by "the other kind" who out-reproduced me and my peers. One way or another, we'll have to stop growing as a species, and one group's excesses have to balanced out by another group's shortfalls.
The only problem with that (if you care about what happens to humanity after you die) is that it will likely result in a major collapse of society. The religious nuts and those in regions of high child mortality (which means they're all poor and uneducated) aren't going to be able to maintain or advance a technological society. They can breed, but that's not enough to know how to keep complex systems running, and things will collapse just like Ancient Rome, with technology being lost and everyone going back to being feudal serfs, or worse.
Personally, I just don't see a way to avoid some kind of major collapse of society due to one or more of the many factors facing us. For this issue, I think greatly improving human longevity is absolutely necessary, and if we don't do that, we're going to have another Dark Ages of some kind.
This is an individual vs. society issue: having few kids is better at the individual level, but for society it's not sustainable unless we can figure out how to significantly extend lifespans (i.e., reduce the death rate in-line with the reduction of the birthrate).