I'm not sure how useful this pricing is for the future, as waymo is currently operating on semi-infinite Google money. If that stops, no doubt the price would change too.

The counterargument would be that the external investors (Sequoia, Andreessen, Fidelity, etc.) presumably priced in this exact risk when they agreed to pay $110B. They're not naive about Alphabet's role as backstop. The question is whether they believe the "semi-infinite money" assumption is durable enough over their investment horizon.