I suppose it’s about minimizing variables, but older AW chargers should be forward compatible with newer models.One of the questions the 'Genius' had in his script was confirmation that I only use the Apple Charger that came with my Series 3 - which I do. He went on to mention as an aside that they even don't want you to use Apple Series 1 or 2 chargers either. Tough crowd!
https://discussions.apple.com/thread/8080610
Series 3 does apparently support more charging profiles / standards as it has been found to work with some Qi chargers. But it’s not like the existing default charger support had been thrown away.