And this makes me wonder why writers don't plan that kind of stuff out. Instead of stretching a 13-episode story arc across 20 episodes, why not tell it in 13 and then come up with another story arc for the back half of the season? Treat the narrative like two separate seasons instead of one long season that you need to fill out.
Er, because you've just massively increased the writers' workload without any pay increase.
I think people forget that this is a job you get paid for, not something you are doing for fun. That's also why hardly anyone is planning out shows seasons in advance. JMS'
Babylon 5 is notable for being the rare exception to that. But the vast, vast majority of shows are only planned out at a very skeletal level when they start, if at all. The showrunner might know how they want to end it, but may have no idea how to get from A to B.
When you begin, you also don't necessarily know if you are going to get 13 episodes, or 20, or 26. Hell, you might get canceled after two! It's very hard to plan in that kind of unpredictable environment. I try to be appreciative of the work that must go into that sort of adaptability.