Because TCs ALWAYS degrade IQ. The more the multiplication factor, the more the degradation. On zooms more than on prime lenses.
I do not own any 2X. I seldom use my TC 14 E, and only when it is strictly necessary.
Well, initial report from Ricci (video posted in the other 70-200/2.8S thread a few times, I don't think I need to repost the link again) actually showed that 70-200/2.8S + the new TC2.0 is not shabby at all, not quite up to 180-400/4 standard but much closer than I expected. I am not so sure that any 100-400 or 80-400 can do that.
I certainly am very interested and once my TC arrives, I do plan to do some comparison with my Sony 100-400GM which I considered to be the best 100-400 currently (from what I see from various reports, it is at least as good as Canon's version if not better). If 70-200/2.8S + TC is as good as I hope, by the time international travel is back to normal and I can resume my postponed Botswana trip, I think D6 + 600/4e FL or 500PF and another Nikon body with 70-200/2.8S + TC might be a better option than A7r IV with 100-400GM.
In the past I did not consider this combination as my TC20e iii was not so great with 70-200/2.8e FL but this time, the situation might be different.
