To be fair, and this is from someone who believes we're never making it to the SB with Dak, Dak wasn't the reason we lost in 2016. He played well in the playoffs. That was more the defense always with it's habit of not making stops in the playoffs. Dak didn't really start his epic playoff choking until after his ankle injury. It broke him mentally.
There's no way to tell how we'd have done with Romo in, but since it wasn't QB play that caused the loss, the defense's lack of stops in critical moments makes it unlikely Romo would have altered the outcome much at all.