In other words, when it becomes clear that the next generation will be worse off than the current generation because there are too many people, it is very likely that anti-fertility memes and possibly laws will become prevalent.

I don't know about laws, but I don't see why “high fertility decreases happiness of humans” necessarily implies “low-fertility–causing memes have a reproductive advantage”. Nor why that advantage would shadow that of other memes.

Insofar as people are mortal, significant changes in fertility and population can happen extremely quickly.

Local (time-wise) changes can be quick and large in any direction, true, but I thought the argument was that as long as fitness selection works, in the long term, it always finally pushes them back towards the Malthusian limit.

So even in an extremely sane and hedonistic population (say, immortals who mostly never choose to reproduce), as long as some inheritable trait (genes, memes or something else) [a] increases the likeliness of reproduction relative to the average and [b] varies among the population, eventually individuals with that trait will dominate because... well, they're the only ones who reproduce.

(Of course, if society can perfectly control all such factors, then the problem disappears. It's just that it doesn't seem possible even in theory.)