Introducing advertising to a subscription service changes what the service is trying to maximise. The commissioning consequences follow directly from that change in measurement.
The two models measure different things
A subscription business earns the same amount whether a member watches one hour a month or two hundred, so its priority is preventing cancellation.
That favours a small number of titles people will join for and stay for, which is the logic that produced the era of expensive limited series.
An advertising business earns per hour of viewing, so every additional hour is additional revenue, and the priority becomes total time spent.
Volume becomes more valuable than prestige
Under an advertising model, a large library of watchable material can outperform a single acclaimed series, because it generates far more hours.
Long-running procedurals, reality formats and older catalogue perform strongly, since viewers return to them repeatedly and watch them for extended periods.
This is why platforms began paying substantial sums for the rights to series that had already run for years elsewhere.
Such acquisitions are also cheap against the cost of originating equivalent hours, since the production risk was carried by somebody else and the outcome is already known.
Advertisers have opinions about context
Buyers select the surroundings for their advertising, and material with difficult content attracts lower rates or is excluded entirely.
Commissioners are aware of this, and a project whose subject makes it hard to sell advertising against carries a disadvantage that has nothing to do with quality.
The effect is gradual rather than dramatic, expressed through what is renewed rather than through anything anyone would state as policy.
The shape of an episode returns
Streaming episodes were free to run to any length and to have no internal structure, because nothing was inserted into them.
Advertising requires break points, and break points work best where the story creates a reason to stay, which is the structure broadcast television developed over decades.
Episodes commissioned for advertising-supported viewing therefore drift back towards consistent running times and identifiable act breaks.
Why measurement got more contested
Advertisers require independently verified figures, since paying against a platform's own count of its own audience is a weak position.
That has forced services which disclosed almost nothing to publish viewing data in comparable formats.
The side effect is that the industry can now see which titles actually perform, and commissioning decisions are increasingly defended with numbers rather than with taste.