It may be to do with writing arcs. After five seasons, things start blurring into each other a little and if you're living from season to season (the way most shows have to these days) rather than from an overarching story arc, then you start to encounter the problem of 'only so many stories to be told' and the difficulty of working out how to develop new ones.
Then again, as shown, it's not the writers who determine what shows keep going.
It might just be that TV channels want new shows for new audiences, rebranding and adjusting their 'franchise' as they go along.
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Then again, as shown, it's not the writers who determine what shows keep going.
It might just be that TV channels want new shows for new audiences, rebranding and adjusting their 'franchise' as they go along.