Not to be argumentative or anything, but this just isn't true.
Supernatural is one of the top rated shows on The CW.
The CW's ratings are down across the board, as I believe is the case with many networks. But Supernatural is still in the top 5 shows on The CW and in close proximity to the rest of that top 5.
While The Flash is clearly the highest rated show, the next four are all very close in the 18-49 demo. After Supernatural comes in at #5, there is a larger drop off. And even then, those shows (iZombie, The 100, The Originals) are still getting renewed on a regular basis.
It's also worth noting that Supernatural has the smallest drop off in the top 5 shows from the previous year. (Supergirl's drop off shouldn't be counted, however, as it aired on CBS previously.)
Unless Wayward Sisters has disastrous ratings, which is doubtful, it should be just fine.