If they solved their bokeh problems, perhaps the lens would have more appeal, but I think that bokeh is a problem inherent in the DO design?
The 400 didn't have a bokeh problem. The 70-300 DO was the lens with the sometimes weird bokeh. The internet seems to have lumped both lenses together as if they were one and the same. Spec highlights on the 400 could have a bit of a bullseye effect but that was about it. The OOF areas aren't as nice as the 300 2.8 or 400 2.8 in my opinion, but they aren't really problematic either.
I think the 400 f4 DO II would be pretty amazing if it were about 2/3rd or 1/2 the cost of a 300 2.8 IS II but that's really unlikely.
That depends on your definitions of "problem" and "bokeh".
I think the 400 MkI does have bokeh problems, I used one for a day and got it to do stuff like this, I am sorry but for >$6,000 I want much better than that. Now I will admit that I personally shot over water like this regularly, so for me it was an unsurmountable issue, and I can well understand others happy and extensive use that never induces such low quality, but for me the 300 f2.8 IS MkI and 1.4TC was a much better, and cheaper, buy and in my opinion DO sucks, yes the 400 DO sucks less than the 70-300 DO (which really sucks) but they both suck.
I'm not seeing any Bokeh or contrast issues with that pic you've posted. Shooting into the light (contre-jour) with sparkly waves regardless of the supertele will pretty much give you what you've posted.
In the image you posted, I would be more concerned with your burnt out highlights than the quality of the Bokeh.
The contrast in this image is particular high and prolly needed better post prod to pull more out of the RAW file.
Careful metering & careful post prod. Notice the quality of the bokeh is very similar and the contrast is less.
These were taken with a 5DIII, ef 400mm f2.8 LIS and with 1.4x or 2x TC's
I choose the far more heavier f2.8 version because it's IQ is a lot better than the f4 DO version (I can use a 2x TC and get amazing sharpness wide open) and I really like the extra stop...sometimes f2.8 is the only way to go!