Someone was complaining about this on the Steam forum, and I'll say the same thing here I said there: It's not like people got stuck with a pre-order price for a game they haven't played yet. Anyone who purchased this game during early access has had an opportunity to play it since then. I purchased mine back during the Christmas holidays and I'm not going to sit here and pretend that wasn't worth whatever would have been saved with a 20% discount today.
In other words, given the choice of 20% discount versus being able to play Divinity : OS for the past 6 months, I would easily choose the latter (and I did).
As for anyone who purchased the game in the last week or so, purchasing any game on Steam that close to the Summer/Winter sale periods before seeing what shows up is generally not a good idea.
