honestly to me this was kind of the best possible development? like we knew they werent going to ride off into the sunset happily ever after; that just doesnt happen this early in the game and honestly i would have felt cheated with that ending. lexa didnt die. she is still alive. it wasnt some tepid “okay yay we won anyway im going to go over here now maybe we’ll see each other again sometime” because we know those always get swept under the rug and never heard from again.
instead what we were given (assuming its face value is true) was something very on par for lexa, something juicy that will have rippling repercussions, both internally for the two of them and externally in their respective forces, and, maybe most importantly, an actual impetus and setup for them to cross paths again in a big way and put a whole new spin on the mutual growth they have embarked upon as two powerhouse characters.
this is how storytelling universally works, though particularly flavoured with the 100’s brand of hard truths & reality in survival and war: you take a genesis point, you compound and convolute, see what you get out of your characters in the arc and find out what they and their relationship(s) are made of, then resolve.