This is an interesting point. For sites like Reddit, inflation is just a (minor) drawback because it means total vote count is a measure of (quality * recency) instead of quality.

But for SO, where voting is never locked and content 'ages' at different rates, it's not so clear there's a downside. The "closed as duplicate" issue still stands, but rising user count is probably helping the problem.

It seems like the rate of site growth and the rate at which you expect answer value to age are independent, so it would be surprising if they were approximately the same and cancelled. And at the very least, SO popularity has to plateau at some point (if it hasn't already).