This post seems ridiculous to me. If you would like to know why your employees are unhappy then why would you ask random strangers on the internet why they are leaving your company? If your (or anyone's) workplace culture discourages employees to air grievances then you aren't entitled to know why they would like to switch companies. Most likely, I think that young people don't wish to be percieved or talked about as whiny (or any other words you can use), in the event that they raise issues which management or colleagues view as unimportant or inconsequential for the company. I'm also curious as to how you know that your Millennial team members are happy, as opposed to working just because they need the money and don't see better opportunities elsewhere.
I think this already exists and is called PeerTube. In my experience it doesn't work very well.