From what I can tell, Witten's 3 or 4 average YAC throughout his career was never good. 1 is brutally bad for that category relatively speaking, but falling to 1 is not significant, particularly given that's never ever been an important part of Witten's game.
And I'd argue YAC for a TE is not a particularly important stat in general.
You know who historically has had a poor YAC? Zach Ertz. In 2016 for instance, Ertz was ranked 241st (!) in YAC. Anyone want to argue that Ertz isn't an effective TE? Perhaps Ertz is showing signs of age at 23 (or whatever he was in 2016)?
Or maybe, just maybe, this YAC derailment is a red herring that has distracted the discussion from the actual point, which is: why did Witten's (and Dez's) numbers dip noticably when they lost Romo?