So a few weeks ago I was taking the trash out to the can for our building, and I saw a vacuum cleaner in it. It looked like it was in good condition, so I pulled it out and took it home; plugged in, works fine.
I opened it up and noticed that the bag was full, and needed to be changed. Today, I finally got replacement vacuum bags and discovered why it had been tossed: Not only was the bag full, there were dust/hair clumps backed up in the tube that feeds the bag, and more clumps built up in the head where it meets the suction hose.
So what I infer is: whoever owned this vacuum before, never ever changed the bag, and got confused when it started to lose suction, so they threw it out and got a new one. Because they have no idea how much junk is on the average carpet, or else they never owned a bag vacuum and were unfamiliar with the concept.
tl;dr I rescued a vacuum cleaner out of the trash, it was completely clogged with dust. I cleaned it and now it works great.