The RF 70-200mm f/2.8L IS was an incredible surprise, and everytime I use it I'm so astonished that no one had made an extending zoom 70-200 before it. It's a perfect lens, there's absolutely no reason why the 70-200 had to be internal zoom--- you wouldn't want your 24-70 to be internal zoom---and when you realize the 70-200 is basically just another 24-70 now, it truly changes how I use my RF 70-200.
WIth that in mind though, I'm blown away that no one thought to shrink the 70-200 F/4 like this before. That lens in itself was always designed for vacation/travel and would have benefited greatly from external zooming. The EF 70-300 F/4-5.6L IS is a great example of that, and a lot of people would choose that lens for its smaller size, over even the 70-200 F/4L IS.
I'm absolutely sure the RF 70-200mm F/4 is going to sell like hotcakes, even if it's around $1,500. It feels so ridiculous when you see how big the competitor's lenses in this class are in comparison to the Canons. If you're hiking into the woods and want a light, compact telephoto, Canon is really going to look like a sweet deal right now.