Not sure how many employees Cisco has left (didn't RTFA), but a quick glance at Wikipedia indicates that they still probably have more than 50K employees. Of course not all of them are engineers, but I have worked in organizations of that size before. If my experience is common (and I believe it is), they have already gone well pas the point you indicate. Even if they only have 5,000 engineers, it is practically impossible to hire anywhere near that many good people. You are stuffed to the rafters with dead wood. Not only that, but quite a lot of that dead wood will have made it up to management level -- the engineering and political skill sets are orthogonal, and people who are good at politics get promoted.
This means that the management probably has absolutely no idea how to separate the good engineers from the bad. In other words, just by growing the company to the size that they have, they are in a position where they can't evaluate talent. A 30-50% turnover rate is another way of saying 2-3 year attrition rate. I agree with the manager. This is common in large companies. Because they are unable to distinguish good from bad, they simply cycle through the available talent in a random fashion. They have chosen to go with quantity over quality and his statement makes absolute sense.
I don't think it is possible to do (for a variety of political reasons), but lets pretend that a company of 5000 engineers could cut back to their top 10% of talant. You'd end up with a solid core of 500 good engineers. Then, let's pretend that you knew how to do whatever it took to keep that talent for 10 year. Would the company be better off? I'm not so sure. I work in a former start up that is trying to scale itself up now. Since I'm fairly senior (possibly indicating I'm better at politics than engineering??? ;-) ) I'm exposed to more of the business end of the company. The CEO is demanding that we double our development group. He knows that this will throw the group into chaos, but he also sees a way to grow the revenue of the company by an order of magnitude if we can do some very specific work. Crucially, it doesn't really matter how badly we do it. It just needs to mostly work.
Which is better? Grow your revenue by an order of magnitude today and destroy your development team, or carefully grow your development team and trust that opportunities will show up when the team is able to handle them? It's a very difficult call. Personally, I can't fault companies who expand quickly (like Cisco did) and who take the opportunities that were presented. That's what the business guys are paid to do.
Luckily, the company I work for is wholly owned by the CEO and he has decided (for now anyway) to adopt a more sustainable growth for the company. Again, I am very lucky that our CEO views our team as being the engine of the company and values long term viability. He doesn't have investors trying to take their money out and has the ability to make the choice that leads to a very good place for me to work. Not every company has that luxury.