Ugh...Burnes was average down the stretch last year, Peralta/Ashby were injured/ineffective most of the 2nd half, Lauer sucked, and they weren't close to being good enough offensively to do anything last year in the playoffs. Having Hader around would've done nothing at all to change those things. Would it have kept Lauer and Williams from pouting? Maybe.
Let's think about what not trading Hader would have the Brewers looking like right now...for starters they wouldn't be leading this division, and there'd be rumors daily about what the Brewers will get for Hader when they trade him at this year's deadline - "if they're lucky, maybe they could wind up with a quality young catcher, an advanced starting pitcher prospect, and some controllable young bullpen pieces...but that'd be too high an asking price for a 2 month rental." Burnes would probably also be on the block, and that once in a generation starting pitching would be gone at least a full season earlier than it will be now.