Hero Academia reaching 20M is huge. 20M is the minimum for a Jump manga to become legend tier.
Let's recall the tiers:
+300 million: Dragon Ball, One Piece
+100 million: Naruto, Kochikame, Slam Dunk, Bleach, JoJo, Hokuto no Ken
+50 million: Kinnikuman, Captain Tsubasa, HunterxHunter, Yu Yu Hakusho, Kenshin, City Hunter, Prince of Tennis, Rokudenashi Blues
+30 million: Saint Seiya, Yu-Gi-Oh, Cobra, Dr.Slump, Dragon Quest Dai no Daiboken, Haikyuu, Kuroko no Basketball, Death Note, Bastard!, Hitman Reborn
+20 million: Assasination Classroom, Eyeshield21, D.Gray-man, Hikaru no Go, Sakigake Otokojuku, Kimagure Orange Road, Toriko, Rookies, My Hero Academia
+10-20 million is the limbo where series like NuBe, I's, Nisekoi, Nurarihyon, Taruruto-kun, Play Ball, Ring ni Kakero, Sket Dance, Bakuman, To Love Ru, Whistle, etc., etc. live. So reaching 20 million is what makes you enter in Olympus.
Right now there are only two contenders for reaching 20m club, and both of them have it difficult in different ways:
-Shokugeki: in 16 million, it has stagnated so hard only chance it has to reach 20 million would be if it doesn't end next 2019. Anyways if it continues its current trend it would reach 20 million in late 2021.
-Video Girl Ai: since its birth in 1989 to 2017 it has been stuck in only 5m (which surprises me). Thanks to live action it suddenly raised to 15M (it raised more in only 1 year than in previous 17 years). But live action is over so I see it difficut to grow again.
Third contender would be I's since is getting live action same style as Video Girl Ai, and it already is in 10m if I recall, but it has it difficult too.
PD: Next year Promised Neverland is reaching the 10m limbo. Black Clover maybe is reaching it too.
PD2: I see Hero Academia reaching the 30m club in 2021 too. Is the very same rhythm Haikyuu has had. In 2021 Hero Academia also would have the same length Haikyuu has right now.