Look at (inflation-adjusted) price then, rather than total sales volume or units.

I mean, sure. I wish you luck in forecasting with that.

Consumer goods prices swinging low relative to inputs, profit ratios falling, or price and quantity moving in ways to suggest weaker demand, should all be determinable with some degree of confidence.

That requires richer data than strictly following sales volumes, of course.

But the basic principle is still one of supply and demand, and following both shifts. The ability of retailers to adjust prices in near-real-time to demand shifts changes the behaviour somewhat. It doesn't demolish the concept of market function.