To expand on my previous post, they've lacked proper vision and talent acquistion for decades.
Before Parcells and Co took over they had Wannstadt following Johnsons departure. The team couldn't draft well. They had a great defensive core, but couldn't find a Quarterback. Their best QB of this century is either Fiedler or Pennington for reference.
Wanny gave up the farm for Ricky Williams who was a great addition until he got suspended multiple times for weed. Then when he was suspended indefinitely/retired the bottom fell out.
Then they hired Saban, and had the Brees/Culpepper debacle which of course they blew. Saban bounced in year two.
They hired Cam Cameron who went 1-15, which led to Parcells and Sparano.
They had early success with the wildcat but their downfall was wasting a draft class on trying to improve the gimmick.
The league figured out the wildcat by year two, and they wasted a second on Patrick White who quickly flamed out.
They had no development from Chad Henne either, which prevented the offense from evolving. Their defensive core was aging and they did hit on Vontaze Davis and Sean Smith to replace Madison and Surtain.
Ireland was given the greenlight to spend, which he did, but failed to hit on the midrange to high free agents. He also didn't have the same draft success as he did in Dallas (Barber, Canty, Ratliff, etc) and of course had the infamous Dez Bryant question.
His final disaster was trading up for Dion Jordan, who was out of place from day 1 for the team's scheme.
With Sparano fired, they got Philbin, a spineless coach who um-ed his way around for years before trying to backstab his QB in Tannehill before getting canned 2 years too late.
Gase had another vision, but grew extremely arrogant from the get go. Got into arguments with Ross and was just an *******.
He led a purge of talent for players who didn't fit his views. Didn't believe in finding good guards and of course the offense struggled to make holes for the run game. Kept his buddy too long at DC which lead to the defense being exploited in the wide 9.
And here we are.
With every coach was a new vision or change in scheme. Sparano brought in the 3-4 which took years to build, and Gase brought back the 4-3(IIRC). They could never find a good QB since Marino. Either failed draft picks, older FA's, or bad trades for QBs. Draft classes that failed and overpaid FAs.