And as far as the ratings decline, the only people they have to blame for it is themselves.
When it was first announced that ABC will have a series based around Agent Coulson's character, the world was afire with anticipation. They got more excited to hear that Joss Whedon was going to be the executive producer.
But no one stopped to think about it and why Agents of SHIELD was doomed.
First off, it sounded pretty cool that Whedon will exec produce something on TV again, but effectively he did the JJ Abrams thing... started a show and moved on, leaving the series in the hands of a trusted writing team. (In his defense, Whedon is busy building a billion dollar movie sequel.)
We all had big hopes, but guess what, this is not Buffy or Angel. This is a show that had a great start, and then was handed off, with canned outlines in place for writers to follow to write their stories within, it showed, and the series was left to flounder.
The things that went wrong and are wrong, are on many different levels.
-Agents of SHIELD, as they said and pitched, is about humans in special circumstances. And that's the problem. It's not the superheroes that people paid money for at the theater to see. SHIELD is the background squad that showed up here and there in each movie. They were the B-team and the executives making tough calls.