The X-Files didn't have a plan to take it into multiple seasons.
Um...that's what we've been saying. That was my example of an
unplanned show. I don't know who you're arguing with here...
You don't have to have the story blocked out into specific episodes, but with the type of series we're talking about (one with a serial element) you need to have some conception of an overall story.
Again, exactly what we're saying, but something you specifically claimed never happens.
How many show creators have said in interviews that they have the entire series carefully crafted out already in advance? A number of them, including Kring from Heroes as I recall.
It's not my fault that you believed Kring when he said that. I could tell from 5 or 6 episodes in that this wouldn't be the case. Everyone was touting Heroes as being everything Lost wasn't (when it comes to reveals and plot development), and I was shocked by that. It was clear that this show had no clue what it was doing.
Some writers and creators are better than others and can keep a series going as if it were all crafted out ahead of time. They take a look at past episodes and factor them in to future ones. Keeping a show cohesive and continuity in check is important and some people just don't have what it takes, even if only to manage this in the hands of others.
I do think that there are shows that don't have a clear end point, and some that don't even have a direction, but have writing teams that can make it seem like they do.
But what I'm arguing is that there are plenty of shows that have their endpoint in place. They have a framework upon which they hang many plotlines that can help them reach their ultimate goal of telling that story, even if they don't know what those plotlines will be when they're 3 seasons out. That doesn't mean that there will be a set number of seasons, because that decision is usually up to the studio execs and the ratings, but having that framework means they can shorten and extend the series if need be to get the story they want to tell in (though sometimes the networks don't give them the chance).