If the show were more efficient with its time the Agreste arc would have lasted one season less at least, and two seasons less if they didn’t want to lollygag at all. I don’t understand the mentality of “our goal is to keep the show going for as long as possible” if quality is sacrificed in the process, which it obviously has.
The answer is just money. You can't sell what you don't have. People cash out of shows at arc-ends often, which risks audiences. It also makes reruns trickier.
It all comes down to money.
It doesn't *have* to hurt the show. Kim Possible was an episodic show that had one single arc over 4 seasons and that arc was just the vague 'Kim and Ron go through high school and end up together' It was just written to work that way. ML is taking a serieal plot and tryig to mangl it into an episodic format.