Yimi Garcia's $6 million salary should have zero bearing on the front office's ability to add more talent to the roster. The tax penalty is basically peanuts as far as actual money outlay in concerned as only overages are actually taxed, and unless a team blows past the final tax level there really isn't much of a penalty beyond a small extra financial outlay. If Garcia is traded then this essentially means replacing one of the team's more effective relievers with a depth arm, surely you can see the folly in such a move. I'd infinitely prefer the depth arms be utilized to cover the team in case of injury and/or underperformance and make the minor league guys force their way onto the roster through effective performance rather than force one of them onto the roster prematurely by trading away a primary reliever.
Garcia is primarily a single inning reliever, and only has a single year of remaining control. He likely only had a few million dollars of excess value. What do you expect that the team would be able to receive in return that actually improves their chances to win vs what Garcia is going to provide? The team is in the middle of a competitive window so it makes no sense to remove important contributors to take flyers on low level prospects at this time.