its not sad end for them they JUST TURNED TO THEIR LIFE AND for me its hinted that they still use the radio and books to send messages to each other so its bittersweet end
the end is perfect it shows that even tho its not fully a sad end but the bittersweet stories are happy even tho they can be sad. and for the last scene we saw YALL I TOLD U ITS BASED ON TRUE STORY even tho i expected a worse end but this is enough for me as long i know that their feelings are still valid and also for me krailert and naran were still secretly sending each other message by radio song choice that's their end for me idc abt how they ended in the show neither about their wives