I have an iphone4 on vzw and I am curious if anyone thinks that carriers will do something to stop allowing us to use older phones on their network thereby causing us to have to upgrade and spend money on new phone. Thoughts?
If the older iPhone is still capable of operating on the carriers system they're likely to let you keep using it as you would be a source of revenue to them. With the iPhone 4, if the 3G system is ever shut down, that will be the end of being able to use it on a cellular network as the iPhone 4 is not LTE capable.
You're also limited to CDMA carriers as the iPhone 4 came in separate GSM and CDMA versions.