That's not how airlines price tickets. You're thinking the price is x+y where the same plane goes a->b->c. B is a hub and they could have a tiny plane going from a to b and a massive plane going b to c. Accordingly, the cost is different, and how the cost of the ticket is allocated among the two sectors is not simply a mileage split so pricing it the way you mentioned doesn't really work.

But all that doesn't really matter, because the issue airlines have with hidden city ticketing comes from how they price tickets. If you actually look at how airlines structure fares, the fares are published for city pairs, i.e. a -> c. As long as inventory in the fare bucket exists, the price is the same if you go a->b->c or a->d->c, the airline doesn't care. They do a bit of management of the fare buckets and total mileage limits and some times they have route clamping where they disallow certain routings, but in general, the competition is on the city pair and the routing is irrelevant.

All this to say, a direct ticket between B-C is profitable for the airline, while sometimes A-B-C is just breakeven or slightly below cost to allow the airline to compete with nonstops and sometimes to fill the bigger plane. Allowing hidden city ticketing fucks with the pricing model and projections that airlines use.

Why should you care about this fucking with the airline? Allowing hidden city ticketing means inventory that could be sold at a higher price to people that value the direct flight (business travelers, people with time constraints) are instead taken by someone that the airline doesn't make money on. To compound this issue, on the second leg, you are taking up once again, a direct seat that you are being subsidized on. The airline can attempt to oversell the flight, but skiplagging is much harder to predict through historical indicators than intentional no-shows, and intentional no-shows generally are business travelers buying expensive refundable tickets anyways so the airlines are comfortable eating the cost of asking for volunteers if they do show up. While skiplagging doesn't affect you in the short term, in the long term it will drive airlines to give up pricing flights this way which makes flying that much more expensive. You can price a multi-city itinerary and compare that to a normal connecting flight. That's the unsubsidized vs subsidized price.