For whom is a declining population a nightmare exactly? I understand why from the state's position of wanting to maximising political and economic influence in the world it might be bad to have a lower population relative to other nations, but I think describing this a "nightmare" is a bit extreme. I doubt anyone in Russia really cares that much if the population is declining or growing in any given year.
Russia is perhaps in a slightly different situation, but my controversial take here would be that in general the policies which are implemented by governments to increase population almost always come with their own negatives. That's not to say there aren't policies which could work, but I think general when leaders become focused on making population line go up they start making decisions which are bad for the average citizen.