Yes, the upcoming season finale weighs on my mind. Let me start of by saying I think the average quality of the writing drops off after season 5. I don't challenge that. But I don't understand:
a) Why 1-5 is such a great story
b) Why the end of Swan Song the right ending for the series?
See, here's the thing. 1-5 in a nutshell, leaving out most MoTW:
John abandons Dean (a full grown adult, don't start with me) to chase down a lead on the monster that killed his wife. Dean worries, finds Sam, which coincides with Azazel leapfrogging Sammy back into the game by fridging his perfect (I do believe) girlfriend just like his mamma got. By the end of the first season, they've all gotten back together, know it's a demon, have a gun that can kill it, but Dean failed to take the kill shot which would also have killed his father.
John trades his soul to hell to save Dean's life and gives away the gun that can kill Azazel. The boys learn even more about demons—about crossroad deals where your soul is collected in ten years but you get something you really want. Sam learns not to love again. We learn Sam has been fed demon blood as a baby (and other babies were fed then, and are being fed now) to prime him for the competition to lead Hell's Army. He's captured for Hell Survivor, and doesn't, but Dean trades all but a year of his life to resurrect him. Jake Talley wins, and goes on to open the Hellgate, letting many demons free on earth. John escapes hell too and helps Dean kill Azazel, and they close the gate before all the demons get out.
They can't work out how to save Dean from Hell. Anything will nullify Sam's life. Dean is savaged by hellhounds and killed, his soul taken downside.
Sam has spent the summer drinking more demon blood and fucking the demon Ruby and trying to work out how to save Dean. He doesn't. An angel does instead, and now the boys are implicated in trying to stop the sequence of events Dean unwittingly started (by caving to torture after 30 years and picking up the knife at Alistair, his captor's, behest) that could free Lucifer. Ruby tricks Sam into pushing the final button. Lucifer is free of his cage in Hell, even though they kill her and Lilith (big player Down Underworld), the mastermind of the whole risky plan whose death is the final piece of the puzzle that unlocks the devil from his prison.
Luci wants Sam because Sam is his meatsuit. Michael wants Dean because Dean is his one true "prom dress". They can take alternatives, but are less powerful and Luci's long term replacement rots throughout the season. Michael wants to convert Earth to paradise (killing half of us), and he needs to zap Lucifer to do it. Lucifer wants to raze the earth (and then kill all the demons, oops) and needs to kill Michael to do it. Other angels have opinions, Castiel has Dean's. They prevent Dean from being Michael's, so he takes their half brother instead. Sam says yes to Luci with the intent of jumping back into the cage and just makes it with Michael/Adam too. Dean decides to live out Sam's final request, drinking like a fish in the house of a woman he had a bendy weekend with and her kid.
Now, I enjoyed myself. But I think 1-3 is a good story, and 4-5 is a good story. 1-5 aren't that great because no one has explained to me what would have happened if Dean hadn't resurrected Sam. Would Jake and his sister be the vessels, even though the Campbells and Winchesters were brought together for precisely this? Would Sam have been resurrected at the start of season 5 so he could be the vessel? Dean probably would have let himself get killed in the interim too, so he'd need to be resurrected too.
If season 5 was inevitable, what were season 1 and 2 for? Kripke admits he got bored of the special kids, so even though more walk among us they're off somewhere undocumented (with the similarly ignored potential major player, Kid Antichrist?) not causing much trouble.
Also, to me, Sam's arc doesn't flow naturally to take him to being hatebanged by the devil for the rest of his life. Similarly, I don't think Dean's doesn't lead him to be a drunkard threat (and inevitable good lay) for a single mother of an impressionable kid. As for us, the world, we aren't better off with neither of them hunting, no matter how many times we skate closer to the edge.
It's an ending. I'll give it that. It's powerful and affecting, I'll admit without hesitation. But other than the erratic decline in quality of episodes afterwards why was that a good place to wrap things up?