here's a theory i cant get out of my head:
WARNING BSD SPOILERS
what if the person that had the page in the first place was oda. think, he never finished writing the book and thats why there's still a page left. i think he intentionally left that page so dazai would put an end to everything. if this is true, i hope dazai writes oda back in and they can ride off into the sunset together with kunikida.