I dont recall AssClass making the charts boring, and it seems unlikely the same would happen for SxF if its anime comes as soon as people want (fall or winter this year), and not at the end of its life cycle where all these legacy volumes would clog the charts like KnY. KnY is, while good for this year, just shows how much money WSJ left on the table for the past few years (consider now its playing catch up with all this merchandise its trying to get out but cant cause Making Product is slow (example, video game on ps4 is coming so late) and you dont know if its a fad, and the loss in all the years worth of Live Events w cast, Radio shows, roadshows, CDs, collaborations with all sorts of media through the years, Stage Plays, Live Actions, like Haikyuu!!, AssClass, and others have gotten throughout their manga lifetimes) it will ultimately be interesting to see if no manga but only an ongoing anime (hopefully????) can keep it afloat like Naruto Shippuden has remained a thing even now (with new arts and merchandise and games) even in the age of Boruto.
SxF could also just crater. (didnt that happen to magi, i recall, incredibly strong start, declined precipitously to irrelevance at the end of its lifetime).