OP talk is funny. Because it'll take AT LEAST 10 years even if it consistently declines to reach the sales that KNY posts right now.
But you must not be following WSJ properly if you think BC is safe despite slowly declining in sales when it's peak isn't even that high. It's "safe" for now but steadier declines will lead to cancellation.
HQ's sales are half of its peak. HQ has a lot more going for it but doesn't mean WSJ won't rush the series to end if the anime's return does not even stabilize the declining sales.
I am, are you following the current Shonen Jump properly? because i think and this isn't just now, that you're a bit stucked in the past when it comes to Shonen Jump, you're still with that mentality of the over ruthless Shonen Jump that would just throw everything in the trash because it's not selling "like it was in the prime".
Black Clover has zero danger for now and for a good while, it doesn't matter if BC is very slowly declining in sales or if the peak it's not that high, the series is a money maker for Shonen Jump, doing great in the rankings, good sales, a long running anime with no signs of stopping and giving good money, a worlwide success, why it would be in danger?
Hell will freeze over before SJ cancel Haikyuu, this series is a money machine and has a big status on Japan, isn't like Bleach that turned into a punch bag in the last arcs, despite the drop in sales the manga's reputation is intact and i can't even imagine the backlash SJ would suffer doing anything against Haikyuu just because its not selling like before anymore.