Baseball is all about tradeoffs;very few teams can afford to have all of the right pieces in place. Do you really want Theo to expend resources on the most overrated, overvalued, and inconsistent pieces of a team (relief pitchers) instead of for better hitting or starting pitching, especially when it likely won't make a single bit of difference this year?
Look at the Cardinals. Their bullpen is arguably worse than the Cubs' (higher WHIP, higher OBP, comparable ERA) but they make up for it with sterling hitting and unbelievable starting pitching and they've got the best record in baseball to show for it. Cubs would be better off shedding the dead wood and expensive players and reinvesting that payroll into a few veteran position players to compliment Rizzo, Castro, Barney(?), Soler, Baez, etc. in the coming seasons. Dumping it into the bullpen would be a mistake.