That Australian licenses it for students doesn’t mean “most universities use it”, nor does the fact that the people you talk to use it mean that nearly everyone uses it,
I can understand that some people, particularly non-native English speakers, will benefit from using something like that. And you’re right, it seems that good proof-readers are a dying breed these days.
However, as nothing but a (long-term) user, I can only say that if it were straightforward to implement KB and the team will have been looking at it; and if Grammarly, why not ProWriting Aid, and Antidote for French and whatever for German, Spanish, Italian, Swedish, Riussian …
Without regard to how many users and where, I can certainly see its usefulness for non-native speakers and writers of English. I can also see its usefulness for students–as a retired professor, I can imagine recommending it to more than a few of my students back in the day. And yet… I know this will sound snobby or arrogant, but for a native English speaker who writes in the language professionally, I can’t help asking , how is this better than actually learning your own language before you write in it? The only use I can think of for Grammarly for such a writer is for them to learn from it what they did not learn from their teachers, or just from reading, until they don’t need to use it anymore.
[Would Grammarly have flagged that last sentence for pronoun/antecedent inconsistency? I intentionally use the colloquial “singular they” for gender inclusivity, and would find Grammarly annoying if it bugged me about it.]
I am intrigued about the plagiarism. You mean it alerts you that you have just plagiarized something? How in heaven’s name would you not already know that? Or does it help you massage borrowed text until it is arguably not plagiarized anymore? Or is it usable by professors and editors to spot plagiarism in writing submitted to them? That I could have used when I was still grading. I never failed to spot the stuff (what can be stolen online can be busted online), but it would have saved me a lot of time.