well many users still have a 3GS and Apple still sells the phone. Even though i believe the 3GS won't get any 6.0.1 or 6.1 update. Apple will stop selling it this September.
I don't think so. The 3GS will stick around until such time as there's a new iPhone (mini?) specifically designed for the sub $200, entry-level market. Apple needs a phone at this price point to compete with low-end and mid-range Android phones.
The iPhone 4 will be dropped, rather than have its price lowered, because it's more expensive to manufacture, and not differentiated enough from the 4S and 5.
And why on earth would they ship 6.0 for the 3GS but not 6.1? Makes no sense.