It was more than obvious that Yacchan was the antagonist of the story.
Yacchan is much more developed than kashima. Kashima is the standard bl top with not much dimension to him. He immediately falls in love with Toono and we dont really know why. It is definitely more likely that Kashima would win only because authors will usually go with the cliche. However because we see Yacchan slowly fall head over heels with Toono and their dynamic is very unique, and knowing the authors other works she is not a fan of cliches so the author could have still made them cannon to deliver a more unique story.
Lmao how did people ever think "yacchan" was gonna be endgame, is this your first time? Read between the lines omg.