Yet, the entire argument rests on believing that life can spontaneously arise and organize itself out of non-life. If that is not possible, then you can have 100 billion, 100 trillion, or 100 zillion chances and it doesn’t matter one bit.
No one's ever proved that life spontaneously arose and organized itself on this planet. I'm afraid that's not the basis for the entire argument that life exists elsewhere in the universe.
In fact, the question of 'how' life arises, is completely irrelevant and non-sequitur to the conversation about life elsewhere in the universe. The sheer scale of the numbers involved with the question, argue for a 100% certainty that life exists, and is even abundant, throughout the universe.
We don't need to know how life arises to examine the question of extraterrestrial life. Our planet is proof that life can and does arise.