My take is the Brewers have been listening to trade offers on Hader for a few years now. I think it's likely they had a good idea what his trade value was from other teams' perspective.
The optics weren't great but if you look at how this team has been playing, and I'm not trying to be a negative downer about it, I can't imagine many fans feel like this is a championship team this season. Anything is possible but I think most fans felt, like I did, that they needed to get help on offense at the deadline to bolster their chances.
Stearns said he wasn't having much success finding a match to acquire a bat for reasonable compensation. I believe him, offense is down across the league so it stands to reason it was a seller's market for offense. I think that's why he pivoted to adding bullpen arms, if they are going to have a harder time scoring then they need the pitching to hold leads whenever possible.
Hader has been a very popular player among Brewers fans, rightly so. It hurts to lose a favorite player. I do feel reactions have been a little over the top though, people have been pretty fatalistic about it. I just don't feel like moving on from a successful closer is quite as big of a deal as it's being made out to be by fans. Williams is more than capable and I feel like Brewers fans know that, so the additions of multiple veteran relief pitchers should in theory enable the team to continue to be just as successful closing games in the late innings as they have been all along.
Sure Hader was elite and was a luxury most teams don't possess but the focus has to be that just because he's gone it doesn't mean the Brewers can't still compete at a high level.