You see, SNK's first half of season 3 aired in 2018 and in that year the series sold 5.2 million copies (it was #3 behind OP and MHA). The second half aired this year and the series sold 4.7 million (it was only #7), with one volume not even selling 1 million before the end of the fiscal year. The newer seasons haven't had any effects on SNK, but also, S4 seemingly won't be made by WIT, which might make the anime's quality drop considerably and not attract much buzz.
As for OP, if you look at the last volume's sales, things haven't been looking that good either. Ever since Wano started back in volume 90, volume 92 and volume 94 have both underperformed, with volume 92 only selling 2 million after 3 months and 94 is on its way to do the same. And while the anime's production has been hyped, I have doubts it could give the manga any boost, given that not even Stampede was able to make it happen.