Why ? The device still works on iOS5 and will still run 99% of the apps available today and over the next few years.
Most developers are still going to be targeting iOS4+ for a while to come.
Eleven good reasons:
mfincham@desktop:~/Desktop$ grep "Subject:" < ios6.eml
Subject: APPLE-SA-2012-09-19-1 iOS 6
mfincham@desktop:~/Desktop$ grep "code execution" < ios6.eml | wc -l
11
Furthermore, "next few years" seems quite optimistic to me. There's no harm in unlocking these unsupported devices, then people can EITHER run iOS 4/5 and live with the flaws OR run their own code. Nobody loses!
(Note as well that the bugs are things like "loading a web page could result in arbitrary code execution").
The moment the device is no longer supported, unlock the bootloader. If you really felt philanthropic, release some docs on the hardware interfaces.
This way Linux / *BSD / Haiku / whatever could be ported and extend the usable life of the device. This would foster a thriving second hand market, massively reduce the security risks that users of the older devices are exposed to, help clear old stock, keep devices out of landfills and so on.
Apple has already said they don't support the device anymore, so they have nothing to lose by opening it up (if we accept that the reason Apple locks down their bootloaders etc is to ease the burden of support from people breaking their own devices, not an argument I entirely buy...).