They're apparently writing Homeland with an end in sight now, next year's season will be the last. Completely agreed though, American networks need to have more integrity in how they plan the long-term future of their shows.
There's a financial incentive, too - if a series goes stale, it spoils its long-term profitability. People are less likely to watch repeats or syndication, less likely to buy DVDs/Blurays, less likely to stay subscribed, and less likely to buy associated merchandise. It also damages the network's brand.
Homeland's a particularly good example of this effect, because it's gone from being one of the best shows on TV - namedropped by President Obama - through to losing its purpose but maintaining quality, then in the last year or two to, as Alionic says, being barely watchable.
