Promos at Google get exponentially harder at each level, which also means the mean time between promos increases as you go up. L9 is Senior Director, meaning you're managing hundreds of people, and hitting that in 10 years is highly unlikely.
The last person I knew who managed thousands of people hit that after 7 years. It's not the norm, but it happens.
And with 15 years of experience I was being groomed to be promoted from an L6 to L7 "Uber Team Lead"... it wasn't easy and wasn't coasting, took a ton of very hard very impactful work... but it's possible for a mere mortal.
