The 46-year-old stands accused of failing to future-proof the company after being unable to secure additional funding from the Government or land a substantial commercial partnership.
While lauded for forays into digital, Duncan has also been criticised for taking his eyes off the ball with ill-fated distractions such as DAB radio.
Pressure has been mounting since Lord Carter's Digital Britain report decided against siphoning money from the TV licence into Channel 4, despite the broadcaster warning of a £150m shortfall in funding by 2012.
Instead, the Government greased the wheels for a potential tie-up with its favoured commercial partner BBC Worldwide, but three months on, there is still no deal.
"His departure was no big surprise," said ZenithOptimedia's head of trading Chris Hayward. "What Channel 4 needs now is someone with a very solid TV base - including a strong programming element - who understands digital."
For his part, speaking to Media Week, Duncan still refuted claims the BBC Worldwide deal has been canned, and repeated comments made by outgoing chairman Luke Johnson, that C4 "continues to travel hopefully". He added that he hoped to have "two or three new commercial partnerships finalised before I go".
While denying rumours he was pushed, Duncan admitted his departure "was a mutual decision" between himself and the C4 board.
"The back of Digital Britain felt like a good time to move on," he said, before stressing the group had "substantially grown reach and market share" during his tenure. Duncan hinted he already had a new role lined up, saying he is taking a new executive position, "which will be revealed in due course".
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