Agreed 100%. Lots of places don't realize that these seniors must have come from somewhere or learnt their craft at some point. They are not born seniors :D
The part of about baggage and preconceptions is definitely true. I'm more productive now than I was years ago but I can also get stuck in a rut faster than before.
Mixing is the key.
Another thing that I think it's very important, which might not apply to Netflix due to size and their "magnetism", is how risky it is to have only senior developers in smaller companies. You increase the bus factor a lot and, as you said, the company is also sending the signal that they don't really care about career development (which might also entail that they don't care about people that much, just the product). So eventually one or some of them will leave. I'm speaking from personal experience here.
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