It's natural, of course, to draw the obvious comparisons with the current crop of Macs--especially the popular laptops--which are in essence sealed boxes, and to tut, don a tinfoil hat, and say that Apple is screwing us by not letting us give our machines the kind of longevity that only upgrading makes possible. I'm not so sure it's relevant, though. For one thing, I suspect many people criticizing the perceived high cost of having Apple replace the battery in a modern laptop not only conveniently overlook the benefits of a non-removable battery, but also crucially have forgotten that the old removable batteries themselves cost only a little less than this service.
More importantly, though, I think we've probably reached a point where a mid-level computer will suffice for most people for a long time, unchanged, thanks in large part to the vast difference flash storage makes to a computer's apparent speed and responsiveness. In the past, you had to keep replacing your Mac regularly because the things we wanted computers to do outstripped their ability to do them.
There's a danger that this kind of talk will come back and bite me like the probably apocryphal Bill Gates quote about 640KB of RAM being enough for anybody, but I think there's a qualitative difference with this situation, and it's this. For the whole of the life of the modern, GUI/WIMP computer--though I'm simplifying dramatically here--we've essentially been asking computers to do the same class of things, and the basic underlying change in hardware since the early days has just been making storage, CPUs, busses, and so on incrementally faster. Now, though, while the fundamental job we ask computers to do hasn't changed--the paradigm remains the same--their hardware has gotten to the point where they can do it without breaking a sweat. Yes, rendering a video might take a while, but the elementary interface stuff of drawing windows and interacting with files no longer has a noticeable overhead.
In other words, until the fundamental role of computers changes--VR? wetware? quantum?--the combination of high-enough clock speeds and, crucially, very fast storage perhaps means that there's enough headroom now; that, in fact, the traditionally prized ability to upgrade components is becoming less of a necessity if what you're trying to do is keep a computer running and relevant for years.
Which beings me to the third reason: My beloved MacBook Pro is running Yosemite, will run El Capitan, and may run further versions of OS X beyond that. There's no Handoff and no AirDrop to and from iOS, but in every important way, it's a completely modern Mac experience. That too, I think, is made possible by how mature and settled OS X has become; Apple can add features and change stuff under the hood, but because the basic business of being an operating system' hasn't fundamentally changed at the same time as hardware has gotten much more capable, even seven year old hardware is still easily capable of running the latest one.
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