I agree with you to an extent. But John, of all the characters, is hardly the most riveting. We are only really interested in him as a result of his friends and especially Sherlock. Without Sherlock, would we find John that much of a character?
Unless there is some sort of distant connection between John and someone interesting that could shake things up a bit.
The fourth season...yes, it will be very difficult to follow on from Season 3. I thought that it would be impossible to follow on from Season 2. I worry. Sherlock in rehab would be something to really hold out for, as well. Fingers crossed.