Creating a patch for all language versions and all different released versions is actually a lot more work than you would think. Most work would go into finding the differences between the new version and all the versions out there, then creating the patch executables (one for every language), and then lots of testing.
So if we do this for English, we'll have to do it for German and French too, and there's lots more than just those three languages who would come here and ask for their fix too.
At the moment, we don't have time or resources to put into that.
Putting together a GOG version as one big download is much easier than updating existing versions on people's machines. GOG is one big setup, not a patch. And GOG is English only. Creating a GOG version was actually less work than it would be creating patches out of it for all versions out there.
We are not forcing people who already have the game into buying this. That was entirely not the idea of releasing on GOG. We have Div 2 out, or it will be out in the near future for US. That might make some people want to play Div 1. Plus, Div has been a pain to get hold of for the last couple of years, so we wanted to reach people who didn't or couldn't pick it up the first time.