There's no reason to suggest that the ducks would keep him up if he struggled for 6 games. Cross that bridge if it comes.
I really don't mean to keep bringing up the Glass comparison cause I'm not trying to make it about him, but Glass didn't struggle until his third line mate Eakin started becoming a turnover and possession nightmare (about ten games into Glass' career) which forced Glass to play a lot of get the puck away from grinders style hockey and his play and confidence tanked before he had his season ended with injuries. The big question with Zegras is can he handle the size and speed of the league at his frame? Even if he comes in as the team's best powerplay option and best all around passer, that doesn't erase the areas of his game that may not be NHL ready, and may hamper his development.
I mean, Z can ride a 6 game hype just fine, earn his spot and still run into developmental issues if he stays. Especially when other teams start ramping up their playoff pushes with faster and more physical play. No one's saying don't be excited about his debut but the Ducks need to be really careful and aware of every element of every game he plays. They can't afford anything less than maximizing Zegras' potential if they want to get to building a strong core for the future. I mean I'd argue Anaheim still needs to draft and develop a few more top 6 quality prospects to build around Zegras. I really don't have faith that guys like Steel, Terry, etc. are gonna reach the level many of us thought they could. Comtois has an outside chance to be a good goal scorer to partner with Zegras but I think the Ducks need more. As it's been said, it sets Anaheim back years if they harm Zegras' development.
He'll be fine. Worse that can happen isn't bad at all: he and the staff see where his game needs tweaks or improvements to maximize his NHL potential and he goes back down to hone it, meanwhile his appetite is further whetted.
Worst that can happen is they see tweaks and improvements needed and figure he's too good for the AHL and he can tweak and hone those needed improvements in an NHL bottom 6 line.