First, people no longer schedule their lives around the schedules that broadcast networks and channels set - where they need to be home each Wednesday night at 8 to watch this week's installment of XYZ series. Folks watch things via streaming at a time that's most convenient for them as the viewer. Many others don't watch their favorite series on a regular or semi-regular basis. They'll spend a rainy Sunday evening on the couch with a bowl of popcorn and a tin of cheezy poofs while binge watching 2 or 3 seasons of a particular series.
Second, if you want people to watch beyond the first few seasons, you need to keep it interesting. With all the competition out there with excellent shows and series on Netflix, Amazon, Max and other paid streamers, if FX wants to make a go of it, they need to ensure story lines are fresh, characters are interesting and that the series doesn't feel worn and mundane in later seasons.
Third, , what Landgraf is belly aching about is nothing new. It's why other national networks put the kibosh on all their original, scripted series as far back as a few years ago. They found it difficult to make a go of it financially and it's not what their modern-day viewers are seeking, for better or worse.