This is definitely going to be fun (NOT) for Firefox users. I don’t know about 80% of existing add-ons not being compatible with the new WebExtensions API, only one of my add-ons is compatible with the new WebExtensions API.
I too can also see this as a potentially massive exit for many firefox users as the add-ons were a very big draw. As we get closer to the date and Mozilla see if developers have converted to the new WebExtensions I wonder if this too will be pushed back.
In the past when Mozilla insisted on signed add-ons and that deadline kept getting pushed back as progress from developers was poor.