The image of the "PAID" stamp made me think that that moment of "payment completed" is the one that no one posts, you only see the final result.

I agree with your idea of what the image represents because in reality, society only sees the final result and never the price that was paid.

Do you think that the majority abandons just before the breakthrough due to lack of conviction or because the price was higher than they calculated at the start?

Again, you hit the nail. Most of leave because at some point, they start doubting if any of it was worth it. Others, just as you've said, may have stopped half because the price now looked higher than what they calculated at the start.