Note: I do not have any information that that is what is happening in Google; I'm just saying what my guess would be if I were presented the situation as "these are the symptoms at Company X. What is the problem?"
It's a pretty well tested principle that massive familial wealth usually disappears by the 3rd or 4th generation (because the descendents regress to the norm in terms of ability) and it looks like we're seeing the same at Google.
They've got too little "real" work for the number of hands they need to keep busy, so unqualified people are going back and "improving" the work of the 1st and 2nd generation.
But because of the company's size, it can't hire high risk shoot from the hip revolutionaries (not that they'd want to work at a buttoned up Corp anyway), so it's staffed with safe employees who don't really resemble the cowboys of the 1st and 2nd generations, and hence can't improve the systems any further.
Happens to most companies beyond a certain size.
They release a new chat app every year and put the previous years app on a 5 year sunset.
So you get to put a bunch of developers on the roof every year.
It's Keynes "break things to put people to work fixing them" at its finest.