this is kind of a dual edged question.
He had a great camp, and he showed he can play in the NHL.
But that being said, while he passed the eye test in his first 9 games, he didn't pass the production test. 1 G, 4 A in 9 games isn't enough to 'force yourself' onto the roster.
he scored 1 highlight goal in game 8 or 9 and they kept him.
There was really no reason not to return him to junior at that point other than 'shiny new toy' syndrome that Adams seems to get from every 1st round pick.
He still could learn from Junior and he'd have more time to physically add muscle that he needs. Burning a year of his ELC to have him underwhelm offensively for extended stretches just simply isn't worth it for his development or the Sabres progression as a team.
In the end, this is another failed example of 'lets the kids play' from the Adams regime. We had cap space and picks to add a forward last summer to help the team, especially in light of Quinns injury. Even more so if you look at the 7M spend on Girgensons/okposo/Jost.
Benson was good, but he didn't make enough of an impact to justify the burned year of ELC and not having a veteran to support the kids already on the roster grow and develop.