Waymo doesn't need to produce a million cars. They need to dominate rideshare in the top 10 US cities, which gives them 90% of the ridehailing market. Then they license their technology into long-haul trucking, then eventually personal cars (with a subscription fee, obviously) to starve out their competitors.
Even 20k cars in one city is likely too many for them.
Tesla's robotaxi solution uses a near-stock Model Y. Tesla has the manufacturing capacity to produce over 5,800 Model Y vehicles per day.
I give them a greater than 1% chance.