Sage is one of the weaker sentinels but on certain maps is very good. I don't agree with you personally just because she does lock down sites with her wall and slow orbs and supports through her utility very well. You might be right as a pure sentinel, shes admittedly weak since her wall can be broken extremely fast and her slow orbs arent good for deterrence because they don't do damage and can only reveal when people are going through them, but on maps like split and bind and even haven where there are sites with one real entrance her wall and slows become a lethal combo for fending off site hits especially paired with mollies.
I think she fits the role of sentinel, the only part where she may be weak at is holding the flank on attack rounds(but even other agents sometimes don't really fill this role that well with agents like Deadlock and her sensors)
In my opinion she doesn't fit the role of controller because the only real piece of utility she has for cutting of areas is her wall and its very limited in its usage because it requires her body to be near in order to place which weakens which areas she can actually control. The main role of the controller is to block off sightlines and make sites more manageable for attackers to take space, and Sage just isn't really good at that. As opposed to someone like omen who can smoke from across the map and has two of them which allows for greater control of a site
respect the opinion though and if you disagree with me I'm happy to respond to you