Regarding the Surface use cases. I use it exactly like you described when in my home office. I have a 27" 4K monitor hooked to the dock, with Bluetooth keyboard and mouse. The Surface Pro is tilted in "studio mode" and attached to the dock. I use the Surface display as primarily a dedicated OneNote tablet ....
Has I've said I'm fairly experienced with the surface computing approach to the modern personal computing model.
What you described did not work that well for me with Surface for several reasons.
1. I have the original Surface dock. Well, in that dock the Surface its fixes in an horizontal position which is not good to write on.
2. Ok. Don't use the dock, just connect Surface to display. Here having two cables, one cable to connect the device to the screen, and another to power limits the way I can position it on the table. I don't draw, I write and in school I've learn to write on a sheet of paper flat on top of the table with one or two hand on top of it, not like e painter in a canvas. So studio mode its un-natural to me when writing. Ok, I could put flat, but has I said, the cables limit my ability to position on the table for comfort note taking ... So it was cumbersome to me. Clearly the system was not designed with this use case in mind.
3. With the iPad with iOS 11 paired with Apple OS High Sierra In can step away from the laptop take the iPad, go to the kitchen as you have said and take everything with me too. So its not a particular Surface centric advantage.
By experience I find useful both visions. The one surface for everything and multiple surfaces per set of use cases. Both overlap at some point but both offer distinct clear advantages in practice. You don't have to think hard to see this. Have a look at the multiple tables in your kitchen, why do you gave multiple tables? Its not just a question of space, but also organisation. Imagine you a table stacked on top of each other, click a button and one table would come up and the other go down, that its basically what MS Surface does. But if you have space to have two tables or more side by side its just more natural and convenient.
What I don't have in the iPad its a PC experience when you have no other option, that is all. The iPad experience its close to match a PC for general productivity but still no cigar yet Apple has to close the gap in terms of interaction language with no reservations to achieve this, people want it rightfully. I think allowing the use of a trackpad on mouse in the iPad its inevitable considering that the display its quite often parallel to the body of the user in this context, and touch in this position its not as practical ... period!! Apple needs to understand this and support at least a trackpad.
Finally, the iPad Pro performance surprised me greatly. Granted, I cannot develop software on it. The only more demanding use case I have from graphical design apps its for my amateur photography activities. I can assure you that its way faster than any other integrated graphics system I've seen, it does not make a noise neither warms up, does not stutter. Unlike other devices that are for some reason or another considered more powerful. I've used both Photoshop and Lightroom, and the iPad Pro counter parts was just way faster, immediate even for my use cases.
Robustness and reliability apart value both approaches greatly. I cannot say that one its better than the other. In that regard, I cannot say that MS its innovating more than Apple at any level. They are following different innovation paths that is all. Each with their own challenges and benefits.
MS its pumping its vision. But if you look at it it’s a vision constrained by their own situation in tech. There is little integration between Windows 10 devices apart from the usual stuff in apps and login, ... uninspiring. Its up to the user to use its brains and be truthful to his experience rather then tangled it with fireworks and potential that has not yet been met.
Personally I find the level of integration that Apple achieved across devices, watch, smartphone, tablet and its PC unparalleled in the industry. A level that you feel information and flows across all this devices as well as use cases. Granted there are some glitches still, but its surprising how well it actually works. I answer and make both GSM calls and messages in any device without even thinking, almost organic. They seam to aim the same kind of organic integration with any medium splashed across these devices. Still there is a lot of space to improve, and this is were I get a bit frustrated with Apple amongst other things. Some glitches are beyond my understanding arriving to the conclusion that sometimes its just lack of care. And in that regard reminds me of MS, with is unexpected, not in a good way. Maybe they are getting too fat and big, ... and greedy.
For instance in the Mac paired with the iPhone when I say to Siri, Call X (using GSM), its not working properly. In order for it to work I need to go to the contacts and start the call there, than it works. But this use case its working on all other devices. Its just an example of glitches I find that cuts the flow, the organic appeal, amongst all others. Mind you, one cannot do this in any other platform, but that should not be an excuse for lack of care.
Cheers.