Throughout college I watched an absolutely insane amount of TV. I would procrastinate the hell out of every problem set and coding assignment I was given, often staying up until 4am or later to binge watch anything I could find. At the time, I certainly thought I had an addiction, and there were moments when I felt like I needed to just get my shit together and be less lazy and all my problems would go away. This never really happened, and I wasn't close to top of my class, but I still managed to graduate without issue and it didn't hurt me in getting internships/jobs.
With the benefit of hindsight, therapy, and over a decade of distance, I can see very clearly that I was actually struggling with some deep mental health issues (among them: depression, ADHD, and CPTSD) and was leaning heavily on TV as a coping mechanism. My parents were deeply involved in my life growing up to a degree that I believe was very detrimental (and their behavior was absolutely one of the direct causes of the mental health struggles that I'm still dealing with today). During college, this “involvement” often meant giving me lectures about how I was in danger of falling behind my peers because I wasn’t interning at Google (and because when I visited home I defaulted to watching more TV).
You say in your post that your child’s grades are “not great”. In the comments, you say they have a 3.0. There is literally nothing wrong with this. Your child’s grades are fine. My grades were fine too, and my parents similarly disagreed.
My take, based solely on the extremely limited information from this post and my heavy personal bias, is that your child is doing the best they can while dealing with something deeper than a gaming disorder. My advice is to let them be. Be supportive, and trust them to handle their work, education, and emotions in whatever way they think is best. It does not at all sound like they are “throwing away a great educational opportunity” (and again, this is basically what my parents said to me in a similar situation).
I apologize if this comes across as hostile — reading this was admittedly somewhat triggering for me, and despite their flaws, I do believe my parents did what they thought was best. At the same time, their best was sorely lacking. In the off chance my take is correct, I believe being supportive without trying to intervene will probably work out in the long run. If I happen to be wrong, it certainly won’t hurt.