Well being proficient on first down is great but that is very often a running play, and very often the yards gained is dictated by the offensive line....as is avoiding negative plays like sacks. See a connection here? Which goes back to why I stand that passer rating is a better individual parameter of performance on third down than conversion rate.
It's not a perfect science, and of course there is blame on Tannehill, blame on the o-line, coaching, run game, etc...
Ultimately it comes down to a subjective decision on whether or not you think Tannehill is good enough. I do....but I am on a "year to year" contract with the guy, which I think is fair. Throwing out 3rd down conversion stats as a damning of #17, is not.
Also how many penalties and drops a team had greatly affects the 3rd conversion rate. How many penalties did we have and how many drops did we have compared to the rest of the league?