Tannehill's cap hit is $3.5M this year, and $4M next year. The team has until May 3 to exercise the 5th year option. The transition tag for QB's this year is $13M, and $15M for next year is fair. The option is not guaranteed, other than by injury, until the player is on the opening day roster of the option year.
The team would have all of next year, barring injury, to decide on keeping the option, a new contract, or walking away. No reason to not exercise it. A new contract after this year could be questionable until we know who will be running the team. A new HC may want to go a different way.