The launcher is currently on a beta phase. If anything goes wrong, we'll simply provide the files as we have done before, with the only difference that there will be more than one file and it will necessarily require 1.13c.
What some peple do not seem to understand is that this announcement tackles "Core changes" those are extremely valuable in order to deliver new content. If you came here to find a patch changelog then it is obviously a dissappointment for you! Anyway those are great news