Hi,
Good reasoning for long lenses. The impression I have that the 200-400 is an excellent lens. Personally, I often use a small pixels APS-C body instead of full frame and 1.4X extender.
Best regards
Erik
The 200-400 is almost universally considered a game-changing lens, both in regards to inter-corporation long lens comparisons, as well as within each individual nature photographer's flexibility and ability to get the shot they want. Pretty much everyone who's used it has been floored by both its image quality as well as the versatility. Haven't seen a single bad review yet, but dozens of stellar reviews.
I have a few pro nature friends in Alaska and other regions, who's work is constantly on FB, who say this lens has opened up new worlds for them, and created opportunities that lens-switching would have lost for them.
For years, the Nikkor 200-400 has made Canon look bad compared to Canon's dinosaur 100-400. Now, compared to the Nikkor equivalent, Canon's answer with its own 200-400 is now considered superior on every level to the Nikkor offering ... from image quality, to range of potential use, as well as by not allowing dust inside (which one Nikon shooter said made him ultimately leave his Nikkor 200-400 home). So, in the lens department, Canon answered Nikkor's once-superiority with an epic rout, eclipsing the Nikkor on all levels.
In fact, the 200-400 is considered by many to Canon's most epic piece of glass, ever, and has pioneered the built-in extender technology, which no one else has added to a zoom. So I am sure we will watch the others play monkey-see, monkey-do in the future.
Therefore, if Canon brings their sensors up to par in the 7D, as well as the next generation full-frames ... in the same game-changing fashion they recently achieved with this remarkable lens ... it will make the wait worthwhile. Even if they just catch-up to what's out there, sensor-wise, they still offer a better deal "whole package"-wise. If not, then I can't see why they would bother upgrading their lenses like they've been doing ... if they're just going to put them overtop of inferior sensors. Would make no sense, so let's wait and see.
Jack