I partially believe that JJK May be benefitting a bit from Demon Slayer, given they’re very similar in overall tone, from the same magazine, and the JJK anime starting the same year that Demon Slayer ended, and right around when the final volume dropped.
I’d imagine a lot of Demon Slayer fans are probably looking for something similar from Jump(this isn’t me saying that JJK will end up having a similar sales boost overall)
Also, it seems like dark fantasy is definitely the huge trend right now, which MHA doesn’t Into like both Demon Slayer and JJK.
Either way though, MHA is still EXTREMELY successful. I hope this doesn’t cause a trend where the anime community clowns the series because 2 younger series surpassed it in sales. The anime/manga community tends to have a habit of doing that