At the moment, you can buy RAM anywhere, so you're not limited to the vendor's overpriced offerings, if they even sell RAM upgrades.
But if Apple came up with RAM units that included on-board Flash backup, they'd be the only source for it, so adding RAM later would depend on Apple's inclination to sell upgrade packages, and would be priced higher than current RAM prices.
At any rate, I don't think that we'll have to worry about upgrading the memory in our computers. The RAM in my phone is soldered in place, and I've never thought once (until now) about adding more. This might seem shocking, but ask yourself: with the rise of the laptop, where have all the expansion slots gone? Why doesn't anyone miss them? They seemed like such a big deal twenty (and even ten) years ago, but now we all get along fine without any of them.
You can talk about how open something is until you're blue in the face, and that has nothing to do with whether most users will want it or not. Walk into an Apple store and count how many customers ask about what kind of RAM is in the white plastic MacBook they're about about to buy, or about Mac OS X's POSIX compliance, or even how to bring up a shell. The number you come up with will average close to zero. What's more, this isn't a bad thing!