I think in the long-run, natural selection will prefer subcultures with higher birth rates, and through that population growth will eventually start rising again. Many of those subcultures are growing exponentially, but coming off quite a low base, so in most countries it is going to take decades, even a century or two, to see the impact at the national and global levels. (Some countries will be seeing it sooner than others–witness the growth of the ultra-Orthodox sector in Israel.)

It will be a rather different population, likely more religious and more conservative, considering that many of the subcultures with higher birth rates are conservative religious subcultures – and, the more conservative and religious a group is, the higher their birth rate tends to be. A society dominated by religious conservatives may produce a slower rate of technological innovation as much as one with a more secular but declining population does. On the other hand, there will always be defections – even highly insular groups such as Old Order Amish and ultra-Orthodox Jews can still have a 5-20% defection rate, and with less insular groups the defection rate tends to be higher – and those defections will help replenish the secular component of the population.

Another factor to note, is that many wealthy countries are avoiding population decline through immigration. That won't change the global population trends, but may redistribute the downsides of population decline towards poorer countries. Given most innovation happens in wealthy countries and relatively little in poorer countries even at present, that may delay for longer any negative impact on technological innovation from declining population.