I recently bought another battery for my Galaxy S2 because the old one died. I have no plans for replacing that phone until I absolutely have to. Preferably not until there is a Samsung Galaxy S6 or so.
But it still works as my bike-phone.
I can understand wanting the software upgrade, but in terms of hardware I do not think it's worth upgrading all the time.
My S2 is still fast enough to run anything I've tried.
What would I get by buying a S4? A bigger screen, 4G access and NFC? Other then that I am not really aware of any benefits. I run CM nightly anyway so in terms of software it's as good as the new phones, and I only ended up running custom firmware because Samsung stopped doing updates.
I can use GPS in airplane mode, so I suppose it works.