Here's my perspective; take it for what it's worth (probably not a lot...)
Determining device longevity primarily relies upon a few factors - applications ecosystem compatibility, task completion speed, frequency of punctuated delay or lag, physical durability, and systemic compatibility.
The first factor is split between software-side and hardware-side dependencies, and has to do with whether the device will remain compatible with operating systems and applications software down the line. My understanding is that Apple has one of the best track records when it comes to support of older devices on the software side, in this respect. 3-4 years is, to date, roughly where they appear to be right now (with the 3GS being at the main tail end, and limited 3G support). I have serious doubts that Apple will drop software support for the iPhone 5's display form factor in the near or even medium-term. The hardware-side dependency is exceptionally user-specific. If you are mainly making phone calls, web browsing, and checking email, something like an iPhone 5 will be able to handle those sorts of tasks perhaps indefinitely when it comes to this particular side of applications support. However, if you are into playing the latest games, then 3-4 years is probably a stretch due to limitations in processing power, and conceivably display support.
The second factor has to do with an aggregated measure of how long the user will have to wait to complete their various tasks. This is, again, user-specific, although most people probably find they "need" to upgrade once aggregated completion times fall below the average needed to complete daily tasks. My guess is, an iPhone 5 will be fine on this measure for 3-4 years.
The third factor is somewhat different. This is how often the device has noticeable (and frequently considered displeasing) cases of lag time in completing a task. While an aggregated measure of completion may show the device as being OK, frequent cases of lag may nonetheless provoke someone to upgrade because it becomes too annoying to deal with an alternation between fluid operation and laggy operation. This sort of thing typically registers as inconvenient, in the minds of users, more often than a unit that is simply steadily slow in the aggregate. This is where I think 3-4 years may become an issue.
The fourth factor, I'm not sure. Such a large change in chassis design, and combined with the relatively thin structure, means it's hard to tell I guess. I'd suspect it'll be OK, provided one exercises adequate care and perhaps uses a case. Battery life will probably deteriorate noticeably within that interval though.
Fifth factor has to do with adequate compatibility with the cellular system, in this case. Since it does have at least the current generation of LTE, it may be OK 3-4 years down the line. Backwards compatibility with current GSM and CDMA networks (depending on your model) also helps. I'd imagine you'd be fine here.
So, in the aggregate, I'd say it'll last but there is significant potential for having things to complain about on the operability front. Then again, I'm not sure there is a manifestly better choice currently on the market. Samsung's S IV will reportedly have a Cortex-A15/A7 big.LITTLE configuration on the SoC, but Android has a less robust track record of software support in the long term.
For the most part, 3-4 years is asking a lot when it comes to a platform. This is usually only possible once a particular type of device has reached a late stage of maturity when it comes to consumer electronics. I'd say we're nearly there with smartphones, but it's not like where it is with personal computers, for example. Still, I suspect an iPhone 5 would probably be at least adequate for that period of time.
As a general matter regarding NFC and fingerprint reading, I can't see how this would relate to longevity. NFC is a marginal technology; nearly as marginal as fingerprint-reading. These features are only peripherally related to the headlining uses of even smartphones, let alone phones in general. NFC will probably receive greater support over time, but it would have to become ubiquitous to place serious pressure on the viability of the iPhone 5 as an everyday-use device. I don't see that happening within 3-4 years.
