Indeed. But airlines don't price solely on distance covered; in many cases, the longer/non-direct route must be sold cheaper than the direct, non-stop service. And in any particular cabin, the person who paid the highest fare may have paid a rate many multiples that of the person who paid the lowest fare. Such a construct can only be maintained if the "extras" (flexibility, for example) that entice people to pay so much more than others for what is esentially the exact same experience (regardless of whether you have the cheapest APEX fare, or bought a full-fare last-minute rate on the day of departure, you end up sitting in the same type of seat on the same plane and receive the same on-board service) - is the ability for them to make the sort of changes to their travel that you now wish to do with your inflexible ticket.

If every ticket becomes flexible - just because - then no-one will pay for that flexibility. Or else, all tickets will have to be sold at higher prices, to account for the flexibility sold with each.

That the plane stops where you now need to be is just coincidence - but, of course, if instead you needed to be in some other place at this time, you wouldn't now be asking if you could step onto a different plane bound for this other destination using this original ticket. Your ticket is good for the stated destination - not for anywhere else (regardless of whether the plane passes through there, or not). The airline asks this of you: to fly all sectors of your ticket, as booked, and in sequence, and completely; if you do not keep to this, then the ticket becomes invalid and is cancelled without refund. This would, of necessity, mean that they will not want to allow you to disembark at the stopover point (but, if you do, make sure you tell the onboard crew and be fully upfront with them, and demonstrate that you accept and realise that this means that all further travel on that ticket is cancelled).

If an airline becomes known for allowing leniency in applying ticket rules, then they will be mercilessly targetted by abusers. Airlines operate on wafer-thin margins as it is. Anything that would undermine them being able to enforce their own rules, or to allow pasengers to behave in ways (while seemingly logical to the genuine traveller) that circumvent the rules would be very bad for business.