I was getting the impression that it was stopping you from zooming in closer.
That would be a bit of a bummer and make the lens not nearly as valuable for landscape. I use my 100-400 for shooting dunes and definitely work throughout the focal range. I guess you would definitely not be using either extender as a result.
Same with some wildlife shots. I don't always need to be at 400. Sometimes you are able to get close enough that the short end of the 100-400 is all you need or you want to add some context to the wildlife shot and shoot the animal/bird wider. Won't be useful for this situation either. Makes me think I will not be pre-ordering the extenders until I think that through a bit more.
After re-reading the product introduction on the B&H site I think you are right. Limiting this lens to either a 420-700 or a 600-1000. Not sure how I feel about that. Adding and removing those extenders in field is not an option most of the time. Definitely either shooting long or over 100-500 range but not both.