I think there's this expectation that online/computer based education would reduce/eliminate teachers and allow for mostly unsupervised learning, and I just don't see how that would work for reading and especially writing.

Maybe reading, a bit, because a lot of people will get most of the way there with learn by example and a couple check ins. And then lots of practice.

Writing seems difficult to computerize. And having a classroom full of students doing roughly the same thing seems a lot less labor intensive than each student at their own pace with a teacher available with enough context to appropriately critique and encourage and provide useful resources.

The arithmetic portions of elementary mathematics seem a lot more amenable to computerization. The computer can easily judge the answers, and while it takes a lot of time and practice to master some of the skills, the breadth of skills is really not that deep, so there's not a whole lot of context that needs to be considered when help is needed. I don't know that this still holds at higher level math, but there's parts that might. On the other hand, group instruction can allow for peer assistance with tricky things which may help both parties understand the material more.