I understand the OP. Read a bit further -- I've never had to replace a computer because of insufficient RAM. The OP needs to replace his computer because it won't run an OS beyond Lion, not because of insufficient RAM.
If Apple had designed the first wave of Intel Macs the way they're designing the current one, then if you didn't buy more than the minimum (256MB in some Macs, 512MB in others), then you wouldn't be able to run Snow Leopard or Lion. My point is that the ability to have the maximum amount of RAM WILL and HAS determined whether or not a machine can run an OS. You can't run Lion on a Late 2006 MacBook if that MacBook isn't running the maximum amount of RAM, for instance.
Minimum RAM requirements doubled 3 years ago, as you and I know, and looking back prior to that it was doubling ever OS release back to Panther in 2003. That's a doubling every two years. That this doubling seems to have at least paused can be credited to stagnating processor advances (we've gone to 64-bit, but is there any demand at all for 128-bit?) and software advances (features being added at slower rates and mostly cosmetic these days).
64-Bit is far from the only advance that has been made in processor technology. Just because 128-bit isn't even worth bringing up, doesn't mean that Apple can't/won't, one day in the not-too-distant future, require a processor feature featured in Sandy Bridge, let's say, that wasn't present in Arandale/Clarkdale. Also, at one new major release a year, you can hardly say that the software is stagnating.
The fact of the matter is that neither you nor I can predict what will cause the minimum system requirements for the next version of OS X to change as they have often changed for reasons that are not immediately apparent and when you least expect it.
Given that, by definition, a machine that allows for the best of everything (especially expansion of things like RAM) will survive the longest before being disallowed to upgrade to the next major release of OS X.
Even if the advancing resumes, it would be 6 years before RAM would be a limitation if you order with 16GB.
Sort of irrelevant, given that the original question isn't "how long will I get?" but rather "what will get me the longest amount of time?".
But more likely than not it will be some other issue that does you in -- consider all the rumors about a switch to ARM processors.
If you believe these rumors, I have an island to sell you at a killer price. ARM has been and continues to be a poor candidate for OS X. It is perfect for iOS as things stand today, but it will never be able to outpace x86 in the ways that x86 outpaced PowerPC, because unlike PowerPC in 2005, x86 isn't about to hit a wall in its roadmap.
Purchasers of the last PowerPC Macs in 2006 were orphaned by Snow Leopard in 2009. And there could be other factors -- as I've mentioned, Mavericks isn't fully functional with Macs not having Bluetooth 4 (aka BluetoothLE) something that most of my Macs in the house, which all run Mavericks, doesn't have. It's pretty safe to say that Macs without Retina Displays won't be supported in a few years -- already there are complaints about system text quality in Mavericks on older displays.
There's nothing about the technology behind OS X that will one day mandate that OS X will only work on retina displays, just as there was never anything in any recent release to ban the use of a CRT monitor. And Apple would be insanely stupid to make such a requirement. By the time that the non-retina versions of the 15" MacBook Pro and 27" iMac are not supported, they will be unsupported for several other reasons that have absolutely nothing to do with the display. That's not how Apple limits minimum system requirements. Odds are that said Macs will be limited due to maximum RAM capacity long before they are limited due to not having a retina display. While I do believe that architectural changes will be more of a factor than RAM when the minimum system requirements next change, to say that not ordering the maximum amount of RAM when you buy a machine you cannot update the RAM in yourself isn't necessary is doing all parties involved a disservice.
So basically, you can put in or order with all the RAM you can and you will probably still find yourself out of support before you are out of RAM.
We got side-tracked talking about RAM. My point is that if we're talking about maximizing longevity in a Mac purchase from a given crop of Macs, we need to analyze all possible elements given that no one yet knows the minimum system requirements for OS X 10.11. RAM is only one of those elements. Given this, let's look at the 2014 mini:
It's limited to dual-core processors, limited to Iris graphics. I'm not saying that you couldn't take the high-end model, stash an SSD, max out the RAM, slap in an i7 over the stock i5 and not be ready to roll for many years. I'm saying that it has just as much of a likelihood of being let out of any major OS upgrade than the 13" MacBook Pros and any of the MacBook Airs do. The 21.5" iMac has less of a likelihood than these machines and the 15" MacBook Pros and 27" iMacs have even less than the 21.5" iMac. The Mac Pro wins this game hands down, but it always does.
So, given this, in order of what will last longer before being let out of an OS update, by definition of every possible thing that COULD (and I say "COULD" and not "WILL") be a limiting factor:
1. Mac Pro
2. 27" iMac
3. 15" MacBook Pro/21.5" iMac (Middle and high-end models only)
4. 13" (Retina) MacBook Pro/MacBook Air/Mac mini (Middle and High-end Models only)
5. Mac mini/21.5" iMac (Low-end models only)
While, I agree that architectural differences will more likely dictate any future cutoff than any differences within the current crop, you don't know that and neither do I, and more importantly, the OP's original question wasn't what will last the longest, but rather, what has THE BEST CHANCE of lasting the longest.
