We would've had his non-guard rights where we could've signed him to a contract up to $18M annually (120%) So yes, we could've kept him.
Expiring contracts aren't valuable unless your trading them to a team looking to create cap space while taking on (likely) mediocre to poor contracts in the process. If everyone is on a fair deal, it's not difficult to match salary as the opposing team likely wouldn't care how many years are remaining on a players contract if it's rrasonable, unless they were trying to shed salary. So Turner is valuable because he could help another team shed salary in 2020? That's a stretch.