I changed the title because there is no way '人類は衰退しました ようせい、しますか?translates to 'Humanity has Declined, Are You Sure?' If this is actually an official Engrish title for the manga then I apologize, you can put it back.
The 'ようせい、しますか' literally translates to 'Fairies, do?' which you could read as 'Shall We Become Fairies?' or more literally, 'Shall We Do Fairies?' which could be 'Shall We Let the Fairies Do It?'. It leaves it kind of vague, so I settled on 'How About Fairies?' which could be read either way, like the original title.
I did a kagi and google search and could find nothing official about 'Are You Sure?' being the real Engrish title, please let me know if that's the case!