The typical ramp up time for a new Google engineer is roughly 3-6 months. So it's more like replace them within 3mo at a generous estimate. Also a few of those were staff You can replace those in 2-3yrs maybe.
Does Google care about this cost? Hard to say from the outside. But for every signature on this you can bet there are many more inside being very vocal. One part of Google's culture is that it encouraged feedback from the engineers and the engineers take advantage of that freedom pretty frequently.
I listened to a longform interview with the head of their HR strategy which referenced the company consistently citing talent attraction and retention as the number one limitation on company growth.
They invest extremely heavily in minimizing attrition and widening their funnel for recruiting. That's why their offices are crazy, they do 3 meals a day for employees, they pay a ton of money for people right out of college and for interns, everyone gets free massages, etc. They take any effect that narrows that funnel or increases attrition very seriously, as they view the company as fundamentally being a stable of the best software engineers they can get competing with the best software engineers other people can get to put out better solutions to problems.
Does Google care about this cost? Hard to say from the outside. But for every signature on this you can bet there are many more inside being very vocal. One part of Google's culture is that it encouraged feedback from the engineers and the engineers take advantage of that freedom pretty frequently.