Hydrogen is the most common element in the universe. So long as you have elemental oxygen, it will react with things and hydrogen is the thing it will react with the most. So having water is almost a given for any Star system. Additionally, protoplanet and cometary collisions are in fact statistically inevitable. The real question is if water can be delivered at a point after enough gravity has amassed to ensure the water stays there.

Right, that's the sticky point? The likelihood of a planet in the Goldilocks zone to be too hot in the early stage of stabilizing its chemistry that it requires seeding with "post-formation" chemistry? Is that likelihood close to 100%, or maybe not even near and we were just set up for a funny cosmic event.