Tannehill is in a somewhat similar spot in Miami to where Sanchez was here after 2011. The locker room is not united behind him and the talent is not good enough to help him win. He's maybe a bit better as a QB than Sanchez was but he's not good enough to really help the Fins get over the top without a lot of help alongside him. The Fins are tied to him for another couple of years or so and the odds on him making that investment worthwhile are probably 50/50 or less.
Tannehill was over-drafted, like Sanchez, and then the guy who pounded the table for him was fired a couple of seasons later. He has a better chance of being successful as a late prime vet on his second team however his ceiling is still probably not very high.
I agree Tannehill is better than McCown. I know some fish fans are unhappy with him, but he's not a bottom tier Qb like some of them think. Miami's problems are mostly elsewhere.