There are a lot of new people, but, when I joined, there were around 75k employees and now it is closer to 150k, so the new people are additive rather than replacements. Only two people I have worked with closely have left the company all together. There have been others in my circles but not really that many and I don't remember any of them. Instead of leaving the company, most people just leave the team for greener pastures elsewhere in the company
Compared to super-early and mid-stage startups, the ability to work a sane amount is done amazingly well at Google, and they pay too much. The downside is that the work is really simple, and the complexity (e.g. of service development) is through the roof and not supported very well. So just get ready to move at less than half the speed you're used to.
TOTALLY worth it to bank money for 3-5 years, IMO.
I couldn't imagine doing such a grind for more than a few weeks.
It's definitely not for everybody, but you might be surprised as to what you find by cutting expenses by what most consider to be "drastic measures" just to see. You can always just go spend more anyways after seeing what it's like!