The lens will sell well. Very well. The 100-400mm is a very popular lens, and this new one is about the same cost, and similarly a variable aperture L zoom. There are people who need 400mm and will buy the 100-400mm. There are people who would prefer a smaller/lighter package and will trade 100mm at the long end for that.
Regarding the 70-300 L vs. the 70-200/4 IS + 1.4x TC, the IQ with a TC will never equal the IQ with native focal length. Using a TC also slows down autofocus. Not to mention that it's a pain to mount and unmount it all the time, unless you're going to leave it on the lens all the time and have a constant 98-280/5.6. In that case, you'd still be better off spending the same amount of money for the new 70-300 L.
If it doesn't do well at that price point, the price will drop. Look at the 60D - price drop, rebate, etc.