The new 70-300mm L is the replacement for the 100-400mm.
I don't agree. As others have pointed out elsewhere on this forum, the extra 100mm makes a huge difference in many ways. Generally, I think these two focal lengths get used in quite different ways.
The current 100-400 is one big lens and not something most people will be taking with them for a hike in the woods. On the other hand, if you want to do any wildlife photography, the extra 100mm can be critical. You can pack a 300mm zoom in your bag, carry it with you and hand hold it under most circumstances. Even with IS, I'd need to spend a lot more time at the gym if I were going to routinely hand hold a 100-400mm and I'm not about to be carrying it on a long hike.
Yes, I would consider a 400mm IS prime, but I've used both primes and zooms and I'd say that even though the prime might be sharper, the ability to zoom can outweigh the sharpness. (And, I am assuming that a new 100-400mm will be sharper than the current model).
I've scratched by head over the 70-300mm L for months. I still don't fully understand it, but I am now willing to concede there may be a niche for it. It seems to have been designed for persons needing an all-purpose zoom that can be carried in the bag or backpack and used under extreme conditions.
I can only speak for myself, but I would say that I would be willing to pay over $2,000 for a new 100-400mm L, but I cringe at the thought of spending $1,500 for the 300mm L zoom.