Am a university professor who frequently assigns essays, so this is something I've been thinking a lot about. The problem is not that AI can successfully write a college-level essay (maybe someday, but certainly GPT-2 isn't even close to that level yet). It's that something along the lines of GPT-2 can emulate the superficial appearance of an essay. The actual facts, quotes, and references it generates are generally nonsensical, but the structure, grammar and language seem authentic at first glance. Unfortunately, a lot of essays in university lecture courses are graded by underpaid TAs who are skimming quickly, and who might be inclined to give an AI-written essay a D or a C- for effort rather than recognizing it as fraud.
The issue isn't so much that AI has replaced essay writing. It's that it has the potential to create convincing (but fundamentally nonsensical/illogical) replicas of academic essays, which have the potential to make an already difficult job of assessment even harder (somewhat analogous to the problem of deepfakes).
By the way, I'm currently thinking about writing something on this topic; if anyone has any thoughts please get in touch. Contact info in my profile.