First of all, there is no "if", unless you think the planet can have 1 trillion people etc.If global population increases are not sustainable, that won't be because I can use a microwave even though I couldn't build one. It will be because there's not enough food for everyone to put in their microwaves. Your two things have nothing to do with one another.
Secondly, since there is an upper limit to population increase, tech progress would either lose pace (like with everything, progressively it becomes more intricate) or a larger percentage of the population will need to create it.
It's easier to see it looking from the other side of time: imagine the tech today, advanced by 1/10 of the scientists able today to advance it: it will go at a much smaller pace. Likewise, when the upper limit of the population is reached, the fraction of it which are advancing tech will have to incease (which is my point), or we will face first a reduction of pace of advancements, and then worse, up to regression.