Zones are unfair because most people only use unlimited travel within
the central part of the system and not at the extremes.
Not true. I have a 456 travelcard and I use it all over these zones.

I knew someone would say that ... I did say "most people" You can have a
system that's fair on you, now can the rest of us have one that's fair on
us too?

I'm sure Oyster could as easily cope with point to point mileage based
Ok then what is the point to point distance for Dartford to London

The rail companies have distance figures and the fares could be worked out
from this data (like they were at one time) - you get the fare online or at
the station, pretty much like anything else you'd buy.

managed to save £400 a year by buying a season ticket for zones 1-4
Well done, but why 1-4, and not 1-2?

Because for some odd reason it was cheaper to do it 1-4 than 1-2.

D