Can't speak for YYZ, but at YVR, flying to the the US, there is no downstairs. On the same level, you check in, drop bags, clear catsa, and then have the choice of global entry, nexus, or the bark of a CBP officer. The only stairs involved are going up a level to use the MLL in the trans border area. Indeed, I am unaware of any pre clearance airport where you go downstairs to immigrate, drop bags, and clear customs. I think you are thinking of the experience when entering Canada at YVR.

Another reason I am happy I was rejected for nexus. For global entry no one cares if my DL is up to date. They barely care if my Canadian passport is up to date; my green card is the sole document I require to enter the US.

As a nexus member you are also a global entry member, and there are global entry kiosks in yyz, YVR, YEG and other stations. Eventually CBP will figure out it is a waste of money to have both forms of kiosks and nexus kiosks will go away at pre clearance. As a someone who frequently crosses the border by car, nexus is useful to you. It is useless to redundant for anyone with global entry since Canadian immigration lines are not known for requiring hours to clear. And more Americans and permanent residents have global entry than nexus. The CBP ultimately has to cater to the bigger market. Nexus for air is toast.

Global entry and nexus exist only because of the inefficiency of the CBP. The only country I've entered that comes close to the CBP in terms of time wasted clearing is India.