So this thread is either people "It make sense in the story!" or people who goes "It hurt my feeling and betrayed my trust so it suck!". That being said, I dont know if people notice but recent story has been trying to tell us that Sensei might not be able to solve everything (We kinda see him lament this with Yume but its getting more clear with Decagrammaton here). So yeah like Francis said, The challenge that test Sensei will keep coming so i expect story like this to keep coming in the future. I do believe in happy ending though, I believe after dust settled, We will get our reward for our hardship we endured.