Battle academy isn't really going anywhere. In fact, we'll get the second seaon of Misfit (2.1 and 2.2), Winter 2022 has Mahouka, Spring 2022 has Mahoutsukai Reimeiki, and we get multiple "Isekai Battle Academy" as well. There are also some greenlit for production with no further information for now. We also still have enough big series in written form.
With anime, the amount simply became smaller, because those "little effort"-anime try to throw out advertisements for the current trend in the hope that some novels will get decent sales out of it. With battle academies, isekai took most of those tropes (harem, OP, comedy, ecchi) and uses them. Many isekai aren't that different from battle academy stories if you break them down to the core parts. And with the switch to more "long-running adaptations" (stuff like shield hero wouldn't have gotten a second season 10 years ago), there isn't really a need to adapt battle academy for now.
But the moment one academy story hit its big again, we'll see a year or two filled with battle academies and tournaments and whatnot.
Although my money would probably be on "isekai dungeon" or "comedy sci-fi" for now.
But as long as isekai are still trending, we'll more rushed isekai with lower quality to make some sales with the popularity. That's it.
If you want to have more battle academy stuff, the best way is always the source material. There is more than enough out there...
