MHA volume sales have always declined between seasons (the only exceptions are volume 21 who came out two months after the season ended, and volume 24, who is the first to show a sales increase between seasons).
Believe actually Shueisha reported numbers, not what OP editor said.
KnY started the year with 3.5M in print, ended it with 20M in print.
What the OP editor said makes zero sense based on externally reported Shueisha numbers, no to mention Oricon.
Yeah, but that doesn't mean that there will be three whole school years.
What if Shigaraki and the PLF attack UA in the middle of that? What if they win?