One cannot compare subsistence living or even starvation, which was the lot of humankind for millenia, with the world we have today. The world population is predicted to rise to 9 billion and then fall off. The bigger problem is where the population fall off is happening, and that’s in all the major economies of the world, which are also those countries that have democratic, humane regimes. The population’s of many autocratic and theocratic countries are growing at a very fast pace compared to the former.
you are getting off into tangents.
you pose an impossible hypothetical. you claim this will cause problems despite the population rising throughout history, and still doing so. then claim it is estimated to rise more, then fall. so based on impossible hypotheticals and “experts” predicting with their models population dips sometime in the future, after 2 billion more people are living than now, that it’s a bad thing some people decide to have pets, but not kids. this is the thing that brings down the entire human race.
unless they are on public welfare, i think it is immoral to tell people they can have no kids, or a certain number but not more, or that they must have kids. i think the best people to determine if and how many kids they have are the two people involved.
you also make the asumption that adults that have pets and love them like kids, will never have kids in the future. many people only first have pets and learn to care and be responsible for another bondable pet - dog, cat, rabbit, etc - as adults. they may not be ready for kids right off the bat. they are the best ones to make their own determination if they are ready for kids, or not.