For me Season 8 was a big improvement over 6 and 7. 10 was a big improvement over 9 (but that's because 9 was terrible). 11 was an improvement over 10. 12 was a sizeable improvement over 11 (though that's because I'm a canon nerd, and 12's continuity was the best the series ever got after Season 5, filling in plot holes and unresolved storylines from Seasons 5-9 and recontextualizing the events of Season 6 in a way that slightly improved that season; the actual story of 12 was a step down from 11 if you shut your brain off for both seasons, but 12 did actually make sense in a way 11 didn't). After that it was basically downhill except that 15 was just slightly better than 14 for me.

Basically the show still has plenty of ups and downs after Season 5, and I think I still agree with the "net positive" thing because most of the later episodes of Supernatural are still entertaining even if they're not up to the standard of the first five seasons. The number of outright bad episodes is less than the number of good or neutral episodes by a long shot.