One of the main reasons for making inclined treadmills is to reduce strain on the knees. A declining treadmill would naturally increase strain on the knees which could lead to serious knee problems. As a result, I don't think this would be a popular item.

The purpose of using decline would be to condition the knees and prepare them for downhill activity. The worse time to discover you could have used more conditioning is in the middle of a 3 hour hike downhill. I had used a treadmill on the steepest incline features to prepare for a hike in the Alps. I had very little trouble going uphill.

It was a different story coming back down. Different muscles are required and of course there is much more stress on the knees. These issues are not addressed by the typical equipment available. If your knees begin to bother you in the gym, you can stop using the feature. Such a choice is not available on a hike unless someone is willing to help you down the mountain side some other way.

I did a search to find out if anyone else had the same thought as I did. It seems there are now a couple commercial quality treadmills available with the decline feature for downhill preparation.

OK, you covinced me. I'm changing my vote.

Actually, you don't need a decline feature. You just need to be able to reverse the direction.