Some wisdom on the modern television series

I was thinking recently that perhaps there ought to be some sort of a rule against television series dragging on for more than two or, I am even tempted to suggest, one season. I am not a big TV watcher, but I flatter myself that maybe that gives me an outsider's objectivity and perspective.
Some of the best series, and here I might mention Lost (now the archetypal the model for many modern TV series) and Breaking Bad, seem to be able to more or less carry off multiple seasons, but even there it is far from effortless. When we get into the realm of second-rate series of the ilk of, shall we say, Arrow, or True Blood, or even, dare I say it, Girls, the original concept, which may be a very good one, gets watered down to such an extent as to be just too thin and diluted to hold the show together.
Self-contained episode series, like Star Trek for example, are obviously exempt from this phenomenon, as are slice-of-life soap operas like Coronation Street (now 55 years old and still going, as characters literally live and die within the all-enfolding medium of the show).
In the ultra-competitive and cut-throat business of modern television, advertising is king, and ratings metrics and audience reactions become the whole raison d'être of a series, artistic integrity be damned. Thus, those series that don't, for whatever reason, grasp the audience's ever-contracting attention, or tap sufficiently into the current Zeitgeist, get summarily cancelled, regardless of where the storyline happens to be at the time. By contrast, those series that can prove their worth in this dog-eat-dog world get artificially extended, often receiving word of an extension towards the end of a season, requiring an abrupt (and often more or less random) plot change or character substitution. This even extends to individual characters: if a character becomes too unpopular among focus groups, they run the risk of being killed off (perhaps we can blame Doctor Who for this idea); a popular character may be given more prominence, or suddenly discover greater hidden depths to their personality.
What gets lost in this process is the old-fashioned idea of story arc. At the start of a series, the developers will usually (although not always) have an idea of how the main story should develop, and probably of how it will end. Suddenly faced with an additional 15 episodes to fill, that story necessarily becomes warped, often beyond recognition. The usual methods employed are unexpected plot twists (although in some shows twists, and double- and triple-twists, becomes the norm, and you begin to expect them), or beefed-up sub-plots, or the dreaded flashback (thank you, Lost!) Eventually, you realize that the main plot of the show has veered off on an unrecognizable tangent to the original one, or that a whole episode has passed without advancing the plot at all. This is usually a good time to stop watching, because you know that there is no way back, and hardly ever is a series able to regain its balance and integrity.
I always think that being a television series producer must be a rather depressing gig. You are entirely at the mercy of the whims of the market, and any artistic vision you may have pretensions to just do not figure in this scenario. But then, I suppose it is a job, and it pays the bills.