/a/ is not your personal club of tastes, shounen threads have been here SINCE /a/ was born, actually, back in the day, /a/ was so lonesome it embraced them, hence the one piece/naruto banners you see up there, people now somehow remembered them now that all of /jp/ content is gone.
shounen is not the cause of /a/ turning into what it is today, its the increasing number of fans from ANY fanbase, its a natural reaction of veteran internet users, you dont want your special place to get raided by new people.
what to do about it? you sadly have to deal with it, /a/ will never be like it was in 2k5, when it only had OS tan, Bridget and fate threads all the day, you just have to move on and try to keep the community alive, not complain about it. that doesnt add anything to the community and further turns it into shit.