Larian really did a mistake with this choice, I think.
Showing themselves as fervent defenders of DRM-free and putting Steam on a box version is a nonsense, and I'm sure they will lose some fans just for that (me first). Even if everybody knows they did it for simplicity reason. They always are good game creators, but to me they're now "just another game devs".
I don't necessarily agree about the outrage but yes, I fear that they crossed a line for many in their fanbase and it was a concern I pointed out when they first mentioned the CE would absolutely need Steam. GOG is not as popular but emphasizing support on it and leaving Steam as the secondary "option" would have been much better for them. Now, moving forward, I hope support for the GOG version will be relatively consistent with the Steam version....even if that means GOG gets one larger update a month vs. biweekly for Steam or something. At this point I think this is absolutely crucial.
