The screen and it's size is only a (very) small part of the iPhone experience.
How could anyone know if they're going to upgrade or not since, even if we assume the screen rumors are true, we have no idea what kind of features the new iPhone will have?
Even though iOS 5 was available and tested for months, very little (outside the microphone icon) was known before it's announcement. I think it's safe to say they might have a similar, but more impressive, trick up their sleeves... but until it's shown, I'll have no idea if I'm going to upgrade or stay with my 4S.