User blog comment:B214/High School DxD EX/@comment-24866657-20150912102407/@comment-26520229-20150920182540

Also remember that EX is an alternate sequence of events resulting from the changes in BorN. Ishibumi was probably referring to the true, LN ending, which for all we know could still be a perfectly happy ending. He never said that alternate timeliness would end happily, but from what it seems like, the kids in EX are trying to alter their history to make it so their ending either aligns with or is compatible with the future in the main story.