I'm going to take this number out of my ass but I'd say that only 1% of the people that watch a video will go read the full description unless there is a specific resource they are looking for(usually links)
You're probably not too far off in that assessment. I find myself going to the description only if the video says there are links to follow in it, or if I really like the music and I want to see if they are credited (which is how I discovered the band Foreign Fields, watching the John Neeman Tools blacksmithing videos).
I either watch YouTube videos using the app on my phone or on my TV using my NVIDIA Shield. I don't see descriptions either way (or annotations, for that matter).
On the phone app, the description is dynamically loaded below the comments. I'm sure there's a less discoverable location, but I'd be hard pressed to identify it.
we will need a firefox extension that reads the description and overlays the annotations on the video at the appropriate time. otherwise it's impossible to do that for any serious video