Over on the Transactions Forum, we've been debating which players to bring in using the farm all winter. Luis Robert? He's a really good player the Brewers could empty the farm for. There were threads about pitching, like the recently traded Jesus Luzardo.
I think that many of us are more comfortable criticizing the front office after they make a move, rather than being critical that they haven't made a move.
For myself, that has a lot to do with the other teams or players involved. Teams don't want to give up their best players. In your Yelich example, the Marlins were having a fire sale. That's the only reason he was available. In terms of free agents, there are many players that simply don't want to play for Milwaukee (like a no-trade list).
Anecdotally, many of us have also noticed a recent trend around the league that MANY teams are hesitant to trade their prospects, not just the Brewers.
In terms of decisions the Brewers have made that bug me? I'm still chafed that they traded away Reese Olson for Daniel Norris, and the board universally disagreed that they didn't protect Shane Smith for the Rule 5 draft.
I think there are a lot of Brewer fans that still bask in the glow of trading for CC Sabathia, and Zack Grienke. Those two moves allowed the Brewers to make the playoffs a couple of seasons. Of course, the Indians got all-star outfielder, Michael Brantley for Sabathia, and the Royals got Lorenzo Cain and Alcides Escobar, defensive stalwarts to their World Series victory.