Screw Britain. How would a 6-hour miniseries help a network like the CW, which has to deliver at least 10 hours of primetime programming per week?
Yeah. You gotta consider the business model that networks (on air or cable) operate under before saying what they should or shouldn't do. What the people who make programming decisions are going to is make the boss happy so they can keep their jobs. Whether or not they "should" be doing this (what does that even mean?) doesn't matter. They are going to do it, for the same reason we all try to make the boss happy so
we can keep our jobs.
So knowing that their goal is to maximize their career and earning potential, what are they likely to do or not do? Try to create hit series that maximize an audience for the longest possible period of time. TV is a hit-driven business. Nobody really knows what's going to work, and most of what they attempt fails, so a few big hits have got to pay for all the flops.
Nobody can afford to deliberately try not to maximize earning potential unless they want to hit the unemployment line. Every show they attempt must be treated as a potential hit until proven otherwise. If it weren't considered a potential hit, nobody would be trying it in the first place.
Even if some network were to say, hey let's do a six-hour show, if the ratings are sufficient, it will be expanded to a longer format. If they ratings are not sufficient, it will be cancelled. Same as if they'd planned for it to be of indefinite duration, so why not just plan for it to be of indefinite duration to begin with and scrap the pretense that it could be otherwise?
This season we saw what happened when a network (NBC) thought they had a better way of programming for primetime.
That's a somewhat different mistake from deliberately limiting the run of a show, but the same sort of mistake - there was no way Leno was ever going to be a hit of the sort that can pay for a lineup of flops that every network has to expect, even ones less lame than NBC. They forgot their business model, which is that if you waste timeslots on shows you know won't be hits, you're never going to get the hits you need to survive. And indeed - NBC has no hits. Idiots.