You have a good point there, My audience retention is low about 15% i know it is bad, even if it's low/bad.
But I still have lots of subscribers watching my videos and coming back watching my future videos and some of them leave comments on my videos so i am getting likes and comments on my videos and it is great because i love interacting with my subscriberd/viewers.
And some of my videos have over 200 views and my other videos have over 100/150 views.
Now i have nearly 300 subscribers(i have 292 subscribers now) on Youtube now.
All so i never ever do sub4sub, i got some people on Twitter saying to can we do sub4sub and i keep rejecting them.
You should really try this yourself if it works for you or not!
As you long you have good enough (great)video content then it will work but if you have bad quality videos then it wont work.