I agree, but there's so many people at Google (132,000 if you can believe the search results), it's hard for me to believe they couldn't devote a small percentage of them to moving to a secure stack.
Let's say 50 000 write code and a small percentage is 10%. So then your idea is that 45 000 would continue to write code in the unsafe languages and 5 000 would rewrite old and newly written code in rust? How many years do you think it would take for the 10% of developers to write all the old code and all the newly written by the 90%?
"Moving to a new stack" implies rewriting much of the code (and: a surprising amount of the code) built on the existing stack.
And you don't realize their codebase must also be really large like their number of employees. They must have a lot of code per employee.