> It's only canceled too soon if the main plot arc isn't resolved, which does happen with some regularity.

When shows become popular, the arc suddenly becomes longer until it loses popularity and they cancel it. There are a ton of shows that have this pattern of having a really interesting premise followed by a pretty fast paced plot moving first season. Then they get renewed for 3 more season and the show slows to a crawl and gets cancelled after season 2.