If you're a total utilitarian, you shouldn't be arguing for population control.
If you're an average utilitarian, you shouldn't be arguing for population control among the well-off. (which I presume bitcoin users are)

But the point is that for long term sustainability, there needs to be some kind of population control in place. In abesence of that, the meme-complexes that encourage child bearing end up populating the majority of the future. These could be fundamental christians or muslims or simply thoughtless people(the idiocracy argument)

For those who think the idiocracy argument is bunk, the  average human IQ has along way to go UP before the idiocracy effect starts showing up. The increase of IQ is due to better living standards and higher birth weights as poverty reduces in China and India.
But the idiocracy effect is real. I live in India, the world's 2nd most populated country. I'm tentative about having more than 1 kid, since i worry about the cost of good school and college. My housemaid already has two kids since she doesn't look that far into the future(atleast not for college) Multiply that by 10s of milliions and multiple generations and you have idiocracy.

The need is real, but the institutions are not in place for generalised human population control. All of humanity is divided in nations that take their individual shrinking as a threat. We humans have behaved abhorably towards each other for centuries. We simply are not at that stage of civilization where we can institute population control and people will comply seeing the "logic" of it. Any general attempt to reduce numbers will be followed by a long round of finger pointing  - you first.

In this stage of human civilization, humanity is what is to be improved and the way to do that is also through humanity. Such appeals are good only for restricting the number of the well read and the  conscientious. So, you will end up restricting the people who can solve the main problem , which for  a lack of a better term, I would term - real long term world peace. And only after humanity attains real long term world peace can we sincerely attempt population control, which will not be followed by backlashes.

If we want population control right now, we need to establish institutions that will lead to peaceful global human interaction.

My favourite means of conducting population control would be tradeable child credits given to every young woman in the world. Let those who want more children buy the credits from those who don't want to have children or have other priorities in life, for which they ahve to sacrifice having children. This  would be possible only after world peace.