sorry for the long winded post here.... but I have a lot to say
I wanted to expand upon my earlier post. Keep in mind that 'future proof' isn't quite the same as it was a couple of years ago..... I'd say that the tipping point was around the Galaxy S3 generation. Before then, most Android phones coming out had some sort of deficiency.... RAM, display, processing power... and Android versions prior to ICS were still kind of a work in progress. Android really started to mature when 4.0.4 was released (along with the S3), then we started getting full featured phones that didn't have "yeah, buts" of generations past.
Mobile apps are simpler than their desktop brethren... you're working on a (relative to full on computers) small display with limited input capability and,
in its current form factor, we aren't really going to see as huge perceptible performance gains with new generations of processors.... Yes, those crazy, 3D graphic heavy apps like Asphalt would benefit, but the most of the time, these phones are running less resource intensive apps that already load quick and run well. We're already at that point now with the SD 800+ phones. Yes, the next gen 64bit SOCs will be faster, push better high end graphics, etc.... but if you are on Twitter, or playing Bubble Witch or something that runs well on the devices now, it's not really going to make any noticeable difference. That isn't to say that they should just up and stop making advancements, it's just that we won't see the hectic jumps forward that we saw in previous years.
The Nexus 5's specs; Snapdragon 800 SOC, 2G Ram, 1080 display.... we're almost a year out and the phone's performance, real-world performance and not just numbers on benchmarks, still puts it on par with any of the newest phones. So hardware-wise, the Nexus 5 should age well. I mean, there are plenty of people still happily using their Nexus 4's, which itself may get brought to Android 5.0(?) by the dev commuity, NOT Google.
The second part is software. That's the trickier portion of this equation. A phone could have the most advanced hardware going, but if the software is kept back (by carrier or OEM), then it becomes a moot point. Luckily, the Nexus 5 has two important things going for it.... First, it is going to get Android 5.0 or whatever they'll tag it, and you'll get it right away... and you'll get quick updates as long as Google supports the 5 (at minimum until the end of next year I'm guessing).
What about post-support? Well, you have two choices if you want to keep it... hold it back on a last-gen OS.... or go custom. Now, to a lot of people, loading a custom ROM might seem like voodoo.... but by the time the Nexus 5 is no longer supported by Google, you will most likely be out of warranty and have little holding you back from loading a custom build. The Nexus 5 isn't locked in any way, and loading a custom ROM is a simple and fairly safe operation. Go watch a video on how to unlock and load a custom ROM, it's easier than most people think. It's hard to really brick a Nexus 5, trust me on that. The Nexus 5 has a very active dev community and wide support, so after Google sets it free, you'll see up to date ROMs for pretty much as long as the Android supports 32bit phones.
OK... done know.