Quantico -Time to end it

This is not a review or rating on this series it's just me using this series as a case study to talk about the mistakes most networks and movie franchises make and yes before I forget Quantico has been cancelled it was not renewed by ABC for another season meaning that after the third season that is currently airing the show will Come to a close, wheew finally a sigh of relief.
Back in the university there was something i came across called the principle of diminishing marginal utility which simply means that after a certain point the returns from a product starts to diminish.
This principle applies a whole lot or if not in totality to the movie industry, because no matter how much we love a movie or series eventually we get tired of the product that is being served to us and when that happens views and online streaming starts to reduce and eventually the show suffers a loss and the show has to be brought to an abrupt end.
Let me start with one of the most popular series the Vampire Diaries, it was and is probably top 10 series if there was ever a list but most of us would not wait for it to conclude. Vampire Diaries got to its peak somewhere between the fourth and sixth season after which it just steadily declined and kept on declining, till the final season truth be told a lot of viewers actually stopped after the 7th season currently the spin off from vampire Diaries which is the originals Is still running I hope they don't make this mistake also.
Also I will talk about the legendary small ville the series about the childhood of Clark Kent and how he became the man of steel. Wow it was interesting, riveting but only up to a certain point  and then it became boring we had too much of Clark by then.
The main reason that this happens is that the longer the show runs the greater the chances that the storyline will get bad,  become stale, hurried, loose its plot which will also in turn lead to a poor output and the viewers are quick to notice it.
I really can't think of much series that got up to an 8,9 seasons that are still attracting much audiences barring game of thrones which is just 10 episodes per season.
Coming finally to Quantico it should have stopped at season two. The storyline was great,  the acting okay, the final episode was good and left to most of us the season and story had ended and on a good note. I was taken aback when season three aired and was I disappointed yes I was really disappointed by what I have seen so far, we have had enough of priyanka Chopra and her co actors,  the season three Is so poor and empty as compared to the first two seasons of the show. They really should have stopped it there, it's always better to leave when the ovation is most high than be pushed away later on when you must have been past your best.
To conclude this network and production crew should know when to stop a series or will avoid spoiling the said series on the long run.  Thanks for reading.