The first Mac laptop to adopt the new "sealed unit" design was the original MacBook Air, and here clearly the groundbreaking, super-slim design drove almost all of the engineering choices. Upgradeability was simply not a priority. I have a first-generation Air (now defunct), and I did end up replacing the original tiny, slow 1.8" drive with a SSD, and I replaced the battery when it failed. While I am fairly well-versed in hardware repairs, I can say that surgery on that laptop was well beyond the capability of most users -- as is now the case for the new Retina MacBook Pro. Many will either replace the entire computer, or make use of comparatively expensive upgrade solutions through third-party vendors.
In the latest iterations of the Mac OS X operating system, there has been a conscious effort to bridge the OS X and iOS worlds, and it is quite possible that this philosophy will be further extended to the hardware lines. Pro users should keep a careful eye on the rest of the product lines as they evolve during the coming year - the 2013 Mac Pro, and to a lesser extent, the iMac and Mac Mini. It seems unlikely that Apple will risk driving away pro users, but it is these models, which do not have the tight design constraints of the laptop line, that will best indicate whether the recent move to "sealed-unit" devices is strictly about design priorities and constraints, or part of a larger shift in philosophy and focus.
The current MacBook Air, like its predecessors dating back to 2008, isn't designed for user upgrades. (Image: Apple)
Ultimately, the question of why the designs have evolved matters less than the bottom line, which is how the changes affect buyers of new Macs, both individually and institutionally. Among other things, it changes the calculus for deciding how to configure a new Mac, especially if you plan to keep it for a while. It used to be that you could get a computer that generally met your needs at the time of purchase, and then upgrade RAM and storage when performance started to drop or OS upgrades required it. With a non-upgradable Mac, that's no longer possible.
My advice: With any sealed-unit Mac, the best bet unless your replacement cycle is short (two to three years) may be to completely max it out with the fully-loaded, most capable build-to-order configuration -- CPU, RAM and storage. Apple has provided a useful upgrade path for adding external storage and other devices via USB 3.0 and Thunderbolt ports, but that does not help when what you really need is a RAM upgrade, nor will you want to lug an external drive around once you max out your internal SSD (though the SD card slot does provide a certain amount of future-proofing). Max it out, or be prepared to replace the whole thing sooner rather than later.
Compared to the old strategy of buying only what you need and upgrading later, this will increase both the short-term cost of the computer and the overall cost. That's because RAM and storage upgrades purchased later when components have generally dropped in price, will need to be bought now, when they are more expensive (and must be bought directly from a single vendor, Apple, instead of whomever has the best component prices).