Ben's contract was eating up the cap space and nobody expected Stephon Tuitt to disappear within a season. They couldn't re-sign Hargrave even if they wanted to. They lost Bud Dupree the next season.
But yes, major gaffe in hindsight. The Steelers are in this spot because they couldn't or refused to face the inevitable after 2019. Ben was approaching the end and didn't want to face the upcoming rebuild.