As someone who prefers reading mode over live preview, this feature is gonna mess things up for me. I often open 2 panes of the same note, one in source mode and the other in reading mode. I then link the two panes so that they will scroll down together when I scroll down.
Sometimes I have to work on different parts of the same note, so having two panes is essential, especially when the note is quite long.
There are already plugins doing this, so I don’t see the need for it to be in core.