Hey,
Even if it increases Alexa, it is not useful for you at all. That is just a third party metric and I cannot see how increasing it would help you.
As for youtube, that is a different story. I am out of the youtube game but 2 years ago it worked. However, it was much easier to just go and buy high retention views. I suppose when it comes to Youtube views retention is still a very important factor.
I can see that there are still services offering high retention views here on the marketplace, maybe you should check thouse out.