Here's my two cents on these two points in bold:
First, I think the idea of a machine being "user-serviceable" is an over exaggeration of what pro users want. I imagine that many pros, in whatever profession or craft they are using their machine for, don't necessarily fit into that computer-savvy image that many people commonly associate them with. Just because these people are working in high tech creative or business fields doesn't necessarily mean they are folks who are comfortable working with hardware or machinery.
I have a close friend who works with several top shelf Adobe creative suite apps on his Mac. I consider him a "pro" because he has had experience in the field working at a start up social gaming company. And although he is well acquainted with all his software, the opposite holds true for his knowledge of hardware. Before he even had a Mac, he used to always go to a friend of his, who would be tasked with custom building a PC for him to do his work on. Other than installing RAM on his iMac today, which lets face it, is dead easy, had never cared to open up his PC to expand PCI cards, graphics cards, hard drives, or memory. In fact, as long as the machine worked, he never cared too deeply about the specs of his machine. Which you might guess why, he later finally got a Mac.
Anyway, I think what pros want and care more than a machine being user-serviceable, is hardware reliability, great warranty protection, and speedy, high quality customer service; all of which Apple is well known for. For someone like my friend, something as simple to us like a hard drive failure or a faulty GPU, would probably freak him out. So instead of having to do the research of finding the right components yourself, all you have to do is take in your machine to your local genius bar and have Apple take care of the rest. Of course they'll experience a bit of downtime, but it's a weight off professional's shoulders to have to worry about the hardware side of things within their machines. The less time tinkering with their machines is more time spent on their craft or profession.
Second, the retina 15" MBP already exceeds the resolution of the 17" MBP, so why keep the 17"? The 15" is smaller and lighter, which I would imagine is better suited for the professional on the go who's always hopping from place to place.