Mark's summed it up nicely. :

I've never felt the need to replace a nut or saddle on an acoustic, because I've only bought guitars that I fell in love with, and part of that affinity was/is how they sounded. Why fix what's not broken?

But, your point #5, tot, eventually popped up on my 37-year old Guild D-40. The nut slots had worn too wide & the saddle had worn away to the point of causing intonation issues. Since I liked how the guitar sounded, I replaced them with the same original stuff: good 'ol Ivoroid. Plastic.

IMO, changing a nut on an acoustic is a pointless excercise of Type A personality : But, as Mark says, saddle material can make a big difference.

Ah....but then there's the bridge pins : Plastic ones will bend & wear out fairly quickly, leading to string breakage at the ball end, or intonation issues from slipping upwards, not holding tight. These I'll change to tusq or ebony, if not right out of the gate, when the originals croak. They're much more durable than plastic and make a little bit of a difference in tone, but not a whole lot. Unless you want to really brighten up the tone of a 'dark-toned' or dead-toned acoustic...then try brass bridge pins for a dramatic change in tone.