Why is Jon still alive?

Beric was brought back to life multiple times. His purpose was saving Arya, after which he died one last time. Melisandre served her purpose and died.

Lord of Light brought Jon back to life. Presumably, his purpose was to gather the army that would defend them against White Walkers. Wasn't his purpose fulfilled as well?

Every other plot related to Lord of Light ended in this episode. Yet when it looked like Jon was about to die, he just walked away.

The defense strategy looked dumb as fuck. It's like the writers never played a single strategy game in their life.

Dothrakis were sacrificed pretty stupidly (although that scene was epic), catapults were placed *in front* of the trenches and fired only once(!), and the archers were just chilling inside the castle while everyone in front died.

It will take some advanced time teavelling miracle from Bran to justify all the characters surviving the impossible.