Building hierarchy into URLs always feels like a neat thing to do. It makes them readable and hackable and human guessable. At least for the subset of people who look at and manipulate URLs. But building in hierarchy is painful when the hierarchies change over time. And the PIPs hierarchy is not stable. Things which were orphans acquire parents as new series get made. So there might be a pilot episode that gets produced and broadcast. If it sinks like stones it will probably remain an orphan. But if the pilot works out, a series will follow and the pilot episode and the series will be wrapped into a brand (or the pilot episode made into episode 1 of the new series).

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So sometimes hierarchies don’t exist and sometimes they change over time. Building them into URLs you expect to be persistent over years if not decades is a pretty bad idea.