The main problem isn't that Mojang can access private chat messages, it's that it is very to get falsely reported. For example:

A: B, what do you think of children?
B: What?
A: Ok, nevermind.
A: How do you get wither skulls from wither skeletons?
B: You kill them.

Person A then reports Person B and only sends the first and last messages, which means that Mojang sees this:

A: B, what do you think of children?
B: You kill them.

Now it looks like Person B is threatening to kill children, so they get banned, even though they didn't do anything wrong. This problem could easily be fixed in many ways, but Mojang ignores any criticism on the chat reporting feature.

Even if people do forget about it, someone will probably get false reported, and then everyone will care again.