The reality is that a "consumer" based boycott, such as that of individuals having less children, will do nothing, just as boycotting McDonald's alone does nothing... or heck, boycotting McDonald's in a large group does very little UNLESS that group addresses the core issues.
The core issues of global demise due to carrying capacity is VERY much due to consumption. When folks talk about overpopulation, they talk about places that are poor and brown. Those are, in fact, where most of the population growth is happening, but those places are hardly the problem. The problem is in places like the UK and the US where a person consumes 36 times more than in Africa. That means an African family could have 34 children and have them live each to 76yrs old and each of them have 34 more children and it would equal the UK or the US.
The problem is that the Global North doesn't take responsibility, and instead passes the buck with folks from the North crying "overpopulation". That means something very different in places like India, China, or Latin America than it does in the UK or the US. In the global south, over population means that there is a lack of basic resources for poor people due to wealth distribution, but not that the earth will burst at the seams with over consumption.
In the Global North, overpopulation doesn't refer to the fact that resources aren't equally distributed in Sub-Sahara Africa (which ironically, is one of the only places on earth that actually lives sustainably), but rather the conversation is about global carrying capacity.
If that is the case, then the UK would have to cut it's population in half. That would mean more people would have to die (probably the old?) to make up the numbers so that the richest can keep up their lifestyles.
IMO, folks that talk about the crisis of overpopulation and how the earth will fall apart due to the large number of people are bamboozled to try to have others take responsibility for the problems of global and local environmental carrying capacity.