Every day we read articles about how AI is going to drive massive improvements in productivity, killing millions of jobs in the process. If we really believe this is true, then a shrinking population is what we want, to "soak up" those destroyed jobs. A bonus of massive productivity increases is that real property transactions as a percentage of the economy will shrink too, softening the blow of contraction there.
Real estate is not the first industry to be threatened by changes in the economy! Creative destruction is part of our economic story and there is no inherent reason real estate should be immune from that, or that we should feel special concern about that industry among all others.
In short, I see no reason to believe that a shrinking population necessarily creates a shrinking economy. "Contraction economics" sounds cool but it's just a new name for what happened to the buggy whip industry, the telegraph industry, the canal industry, etc. And yes, maybe the real estate industry too.
EDIT to add: In my opinion a more significant future for economics is accounting for externalities that have typically stayed off the books, like clean water, clean air, a stable climate, etc. Economics has typically modeled economies as open systems, that is, there are "inputs" like raw materials and energy sources, that create economic outputs. But we've conquered the Earth now... with the exception of insolation, the global economy is a closed system. Every ounce of raw material and every joule of energy creates a complementary cost somewhere else in the system. Unless we can account for that, we will fool ourselves about the actual net value we are creating.