Yes, of course. Valve is a very good open source citizen. The fork was necessary because we have a fair number of changes that are not appropriate for upstream Wine. As a single example, we integrate closely with the Steam for Linux client for stuff like achievements and multiplayer, which obviously can't go upstream. I'm actually the guy who does the rebases, so I'm very motivated to keep the diff as small as possible :) You might find this article from March interesting: https://www.codeweavers.com/about/blogs/aeikum/2019/3/27/how...
Thanks for the amazing work you guys are doing! I'm keeping an eye on the progress and it's amazing to see: https://boilingsteam.com/proton-keeps-making-its-way-in-comp...
I'm not informed enough to say whether the fork made sense or not, but my immediate reaction was that using a OS project with immense man hours and effort behind it for commercial gain without contributing back would be very ... questionable.