two other aspects: creators and screenwriters who don't know where (or when) their own product will end

this is probably the main point. GoT has not good excuse for this because they were well aware that the thing had to end but just mucked it up anyway. But for the most part I think that series continuing has everything to do with audience numbers, focus groups, and profitability. LOST went on for far too long and only threw in the towel once it was no longer making tons of money. The last several seasons made absolutely zero sense and the story from the start of it started to unravel into "WTF is going on?" after just halfway through the 2nd season.

Breaking Bad I think did set the bar really high, because if they had milked it, they could have kept that thing going for several more years and still raked in the money but they decided to have an epic ending instead.

Insisting that series be a certain number of episodes is also a downfall. Some series that I really wanted to do well were destroyed by having 50% (at least) of every episode being "filler" that served no purpose other than to fill up the obligatory 44 minute episode. Punisher comes to mind the most in this example, but there are droves of others.