I finally realized what this last season of Supernatural feels like...if you've watched Star Trek Next Gen or later--
You're in the holodeck in an opulent garden. Not only is it visually stunning, but the texture of the ground underfoot and the plants you touch, the scents, the breeze--all combine to convince you that you are in this astounding place. You walk around in it for hours, immersed in the illusion.
After a while (5 seasons or so) you start to notice there are now tiny glitches--barely perceptible moments where the feel of something is just a little off, or when you take a step something flickers and you are taking that step a second time. But you ignore them and keep walking, still lost in the overall astonishment of this holodeck creation.
Now and then you pass a tree and see an incongruity--a snowman, for instance, but it vanishes as you take another look and maybe it was your imagination, you tell yourself.
The longer you stay you start to see patches where the illusion fades momentarily and you can see the gridlines of the holodeck through the flora. At first the faintness solidifies quickly when you notice it, but eventually (say around season 12) the thin patches remain once you spot them.
And they are getting bigger. But you look away from them because you want to keep the beauty of the illusion going.
At this point (season 15) you see places where the the image of the beautiful garden has simply disappeared and the harsh lines of the holodeck stand out to remind you this was pretend all along. You carefully walk a path that keeps you in the garden but while your feet stay in the illusion, everywhere you look you see bigger patches of the grid's reality.
You know that very soon the last of the garden will be gone. You wish the whole thing had simply vanished in one fell swoop so that the image left in your mind would only be of the initial perfection, but sadly the distorted picture will also be remembered when you recall your walk.
I was trying to explain to myself why it feels sad this year when you see things in an episode that are clearly the writers going 'Dean needs to not have his phone so he doesn't race back to the bunker. Have the bar girl collect it' and then dash off to write the fun/dramatic scene they want to get to without taking time to think through a logical way for Dean to be incommunicado because they are just focused on the big parts of the episode, having forgotten that it was the solid grounding of the details that made the show work in the beginning.