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
Those bugs will never be fixed for iPad 1 users.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").
Most developers are still going to be targeting iOS4+ for a while to come.