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I don't think a manager does either - Google will be dead before they can climb the hierarchy. You think so? Why do you think Google will be dead in the next few years? I'd bet on it being around, in some form, even 50 years from now. A halfway competent manager at Google will make Director in 5 years and VP in 10. That might be, in part, because Google has so few competent managers that a "5" or a "6" is a local 9.7…
I don't think Google will be dead in the next few years. I think it will be dead in about 15-20 (because that is the typical lifespan of a tech company - witness Sun, SGI, DEC, Apple if Steve Jobs had not returned, Compaq, Microsoft, and many others). I also think that the average manager at Google will never reach Director, and indeed a SWE3 has a better chance of making Distinguished Engineer than a manager does of…
Managers benefit from a seniority system at places like Google because you get automatic level bumps after so long. Sure, the ladder will get longer at the top (there will be VPs and SVPs and EVPs and SEVPs and CVPs) as they have to invent new levels up there, but the fact is that it's just much easier for you if you're a Google manager with guaranteed promotions for average performance, as opposed to an engineer who has to do something special to get Principal+.
You could be right, though. Google may have to do an honest layoff (as opposed to the dishonest stack-ranking/PIP bullshit which is a layoff masked as a "low-performer initiative") and, when that happens, a lot of managers are going to find themselves on severance packages. The ones who manage to hold on will still be getting the guaranteed title bump every 2-3 years, unlike engineers to whom no guarantee is made, but it won't be the same, and being a manager at Google won't be the cushy, write-your-own-performance-reviews job that it historically has been. The churn will free up positions, but the game will (as you noted) feel more zero-sum than it is when it's expansion that creates them.