Existing customers are always eligible for the same offers as new ones. Anyone that wants to get a new line BOGO offer can get it
Not everyone needs 2 phones or if they do the 2nd one requires a new line and most people do not need a new line. So basically that means opening a new line and closing an old one which means changing a number you may have had for years.
Even deals where you can get $300 off a phone as long as you do device payments for 24 months, and if you leave early there is financial penalty sounds an awful lot like the old 2 year contracts they claim to have gotten rid of.