We're only talking about the past to help us predict the future. We're interested in finding the answer to the question "Would it be utterly daft to invest in property now (at what we presume is a peak), or can we be confident that our ruler will eventually slope up so long as we make it long enough?".
Why not? It depends on what the major wavelengths are. And of course the further away in time we get from now, the less clear it becomes what the most sensible way to define "real prices" is.
A century isn't long-term enough. Inflation cycles are pretty long. You need more than just a few days' worth of data to be able to measure mean sea level accurately, given that tides have some pretty long cycles too.