in the past we were fortunate to get 4 to 5 good solid consistently well written seasons out of a show. Nowadays we are lucky if we get one,,,I think a lot of that has to do with networks picking up more and more shows that are better suited to be a one season limited run series than a multiyear series yet trying to force them to go longer so we get a lot of stallung and filler. Shows like Heroes and bsg vampire diaries etc proved that,,,one excellent season then the show comes off the rails.
I also think that writers have come to rely on twists, teasers and cliffhangers rather than using those elements sparingly and creating satisfying payoffs.
I would much rather have a show with a much more open premise, modest ensemble so it could tell season long arcs for a few good years than some unnecessarily convoluted mythology show with dozens of characters to keep track of.
and re: supernatural,,,its last great year was season 4. Season five was highly uneven but managed to tell some strong stories like abandon all hope. Season 6 7 8 are wretched.