Forcing their employees to learn rust doesn't mean Google has the capacity to rewrite all their software in rust. They have tons and tons of code which would need to be rewritten from scratch.
Of course if they dropped all other development and told their employees to rewrite to rust, they may end up with a piece of software written in rust but no customers.
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%?
Of course if they dropped all other development and told their employees to rewrite to rust, they may end up with a piece of software written in rust but no customers.