What difference does it make if they are from the past. They are produced and shipped today, components have to be updated, they are sold today, their supply chains have to be managed today, iOS has to run on them today, they get the same warranty and customer support that 6 and 6 plus have. Right now, 75% of their phones are NOT 6 or 6 plus. It takes people and focus to do all of this.

EDIT: The way you are talking about 4s it would seem Apple only has a design team, once something is designed everything else happens automatically.