I was booked to fly IOM/LCY then LHR/HND the next day return on one ticket in "advance purchase" F (known as A for some reason, still relatively expensive and changeable for a fee). If something went wrong with the first leg I was relying on the airline to get me to Tokyo on another flight. In Tokyo I would join a photo tour departing HND two days after my arrival so some tuck was built into the schedule. The weather in LCY was looking iffy for the day of departure so two days before I called the airline to see whether I could move the IOM/LCY flight forward a day. Yes, the kind lady said but the ticket would need to be repriced, and you can guess what that will cost. And of course to make my own arrangements to get to London and not fly the first sector would invalidate the whole ticket.
As it happened all went well (as it usually does....). After holding for an hour to allow a snow storm to go through we landed on a thankfully wet runway at LCY. But I cannot understand the airline's unwillingness to accommodate a change which would, in the event of a problem, make life easier for them. And indeed were I able to not use my first sector and tell them so, give them an opportunity to resell the seat. And I was willing to pay them for another seat the day before....
What the airlines hated was people booking return tickets and throwing away one or more the return sectors to get the overall trip price down. What we are discussing here is no show for outbound resulting in then cancelling the inbound. Yes flexibility to change flights comes at a significantly greater price, but we are not talking about flexibility. If a passenger (sorry "customer") communicates the intention to give up a paid for sector outbound to the obvious advantage of the airline concerned why should the inbound sector be automatically cancelled?