The 300mm lens quality was never an issue. It isn't a large jump from the 70-200. With a 1.4 teleconverter the 70-200 f/2.8 becomes a 300 f/4 AND it has zoom. If you are looking at a 300mm f/2.8 then the analysis is different.
I find the 200-400 zoom to be very helpful. I own the 600mm f/4 as well. By time I'm at 600mm the subject changes to wildlife so I don't require zoom.