It isn't over it's capacity but I also don't think declining birthrates should be considered a problem unless the total amount of people begins to decline at a significant rate. Birthrates only need to be high so that capitalist will have more people from which they can extract capital. If the population stagnates it's not possible for a business to expand indefinitely and investors do not like when they can't profit.

I'm sure there is much more that can be expanded on here but this is a personal belief I just considered and wrote on the spot.

Edit: having young people is important to a communist society as well and a age distribution with a fat middle can be a problem there too because young people do the majority of the work but i think with current technological advances this issue may be and become less significant with time.