I keep hearing people saying "Taking out the 3.5mm socket will allow them to make the phone thinner!". The socket *isn't* the limiting factor on the thickness of the iPhone. The iPod Touch is already 1mm thinner than the 6S, and 1.2mm thinner than the 6S Plus, and it has a 3.5mm socket in it!
"Ah, but it means they can make the phone waterproof!" Other phones that are "waterproof" (as in can survive being dropped into a full sink without an issue) that are already out still there use 3.5mm sockets, so that removes that argument.
"Think of the sound quality that you will be able to get!" As others have pointed out, the majority of iPhone users, particularly those who ripped all their CDs into iTunes using defaults, or are subscribed to Apple Music or Spotify or whatever will see no benefit. There's nothing to stop audiophiles from using the lightning connector already.
"It moves us to the new digital standard!" Well, no it doesn't. If any connector can be classed as a standard, then that is USB C, not Lightning. Headphone manufacturers hate this idea of having to support multiple formats of connection. It makes te planning and logistics that much more inefficient and eats into their budgets.
"Apple have lead the way before! Remember they removed the floppy disc drive from the iMac". True, but they moved to another industry standard, not a proprietary system (see previous point).
There are already options available for those wanting to have the "audiophile experience". Removing the 3.5mm socket and relying solely on the lightning connector is a step in the wrong direction.