Even when airline agents doing stupid things isn't of relevance, there are times when no-showing on a ticket's first segment(s) haven't led to the rest of the booking's confirmed flights being cancelled. When the entire eticketed booking has had flights only marketed-and-operated by a single carrier which is a member of the major 3 airline alliance carriers, then the chances of skipping a coupon or two without it causing issues is even more abysmal than the kind of situation that allowed me to fly just the latter portion of the DL tickets without jumping through any hoops about repricing or getting the airline/TA reps to adjust the ticketed bookings. In my case, it was all SkyTeam marketed-and-operated flights on the tickets (006 stock); but the operating carriers were mixed: DL-operated flights in one direction and AF/KL-operated initial flights in the other direction. The first time I showed up doing this on non-show tickers of this sort was at CPH on a cheap roundtrip TATL ticket; and even after having done OLCI, I fully expected to have issues at the gate at CPH on my US-bound part of the ticket and/or with the airline at the connecting gate for my long-haul flight. But after it worked that first time, the next times I just assumed that with a boarding pass in hand I was good to go even after having no-showed on the first half of the tickets. But that was on some very cheap DL economy class tickets that were bought with the idea of being basically use it or lose it -- fortunately for me, they turned out to be more useful than I expected.