I was initially quite excited but the $1500 version doesn't have a dedicated GPU. It's still a pretty sweet laptop that should outperform a MBP, but then a MBP is $1300. That's a justified price difference I think, the SB has touch, pen and tablet functionality, too, plus better performance (MS claims), for $200, that's not a bad deal (unlike the iPad Pro, the pen - and obviously the keyboard - is included in the price!). Still, if you actually want the GPU you'd have to pay up $1900, at that point it gets kinda steep although that's true for the competition as well, plus your storage is doubled together with the GPU for that price. (can we all laugh at the MBP 15' at $2k that doesn't have a dedicated GPU for a moment :p?)
I totally agree with the MBP's value not being fully justified, at least until recently. I felt like that for years (~15 years of Windows user here), until I recently needed OS X for work and looked at the market more closely. I find that where I live (which reflects the international market pretty well), if you buy a $1500 Windows laptop, 4 years later people look at it as if it has no value and offer like $200 for it. For a MBP, it's more like $600. These things just retain value incredibly well and I'm stupefied to see some of the prices, e.g. yesterday I saw a guy ask $800 for a 2010 MBP (yeah the default one without an SSD, 4gb of ram, an intel core duo and 1280x800 display), it's just ridiculous but people actually buy those things. A 5 year old laptop - i.e. battery is shit and parts could start dying within 1-2 years - that has worse specs than brand new laptops at the same price, yet people buy it, in this case for something like 50% the original price, that's just ridiculous. Reminds me of that time I bought an iPod touch for music (2nd hand), and 18 months later sold it for the same amount, basically had a free mp3 player for a while. These things retaining value means a ton for my buying decision.
That fact right there means I'm, in effect, comparing a $1500 MBP to a $1500 Windows laptop, only the MBP in comparison would only cost me $1100 compared to the full $1500 for the Windows laptop. The Dell XPS 13 non-touch for example is $1k and that has a lower resolution, worse battery, worse cpu/gpu and half the storage, obviously, to a $1300 MBP, yet given the resale value of the two the MBP's lifetime value should be cheaper for the average person. That's why I ultimately bought the MBP (mostly for work, but also because taking into account the 2nd hand selling value shifted the economics such that it made the price justifiable)
The Surface Book feels like it might be different as it's got potential to become a solid 'real brand' that should retain value for quite some time. If I put a Dell XPS 13 on a second hand website, most people considering to buy a 2nd hand laptop (like my dad) haven't a clue that it's an amazing machine, especially 4 years after launch, because it's not a distinct enough brand. But even my dad who has never used an Apple device in his life thinks Apple means quality and knows about the Macbook brand and would pay up for one. The Surface Book seems to me to have a shot of becoming such a brand that everyone heard about and knows works really well and that could be sold for at least 30%-40% four years after launch like Macbooks. But apart from that, the economics of windows second hand laptops never looked good, maybe with the alienware/razer series as a niche exception.
I think you're right on switching out the base, that must be possible if MS wanted to make it possible, but that may be unlikely. One is that it's going to be much more expensive as the base probably has battery packs, cooling and a keyboard in there, alongside of course the actual casing. So for every GPU you buy, you're also buying all of that. With economics like that, it'd probably be a niche demand for a few people who are rich enough to do that, that MS won't be bothered to cater to, as these people are rich enough to buy a new device, too. Two is that cooling & power in such a small and enclosed device might become an issue when you bump up the card, although part of that (better cooling, more battery) is probably in the base so it could be co-upgraded, but that goes back to expense. And three is that it might not make a ton of sense. i.e. the on-board GPU of a 6th gen intel CPU is already pretty damn good, powers 4K, photo and video editing etc. So any dedicated card they do put in should probably be really good, and any upgrade of that that's worth ditching that, plus buying an even better card, plus the rest of an entire new base (keyboard, battery packs etc), would probably then cause CPU to be a bottleneck for practical applications. (i.e. when the CPU is made for the onboard GPU, then the GPU becomes dedicated, an upgrade of that dedicated GPU might create too much of an imbalance where the GPU outperforms the CPU and the latter bottlenecks the former.) Finally it doesn't feel like companies are excited to facilitate this usergroup much, everything seems to be moving towards non-exchangeable.
What if it worked the other way around though... Think about it, the default model's base has a keyboard and some battery packs and no GPU. Now what if I wanted to buy the Surface Book 2? Well nevermind the base, I just want to buy the new tablet and put it on my old base, then give my old tablet to my kid, a friend, sell it 2nd hand. That way you can upgrade the screen, the cpu, gpu, the ram, the storage etc, while using the same keyboard and battery packs. There are some reasons why this won't happen but this might make more sense to a larger amount of users than the niche who wants to switch out their laptop's dedicated GPU every few years.