Yes, we migrated over to Linux for a number of reasons, among them 1/higher developer mindshare, and 2/Linux had better SMP performance for the first decade or so vs. FreeBSD as we started to buy beefier multi-core and multi-socket servers.

I thought it was Marisa Meyer that forced the change?

No, this process began long before she joined the company.

Sigh. I often think an erosion of tech stack also brings erosion of company culture and value.

Having worked at both Yahoo and Amazon, I don't think that's necessarily true. Amazon has a strong culture that's reflected in our Leadership Principles and has survived multiple significant tech stack changes over its history. Yahoo was never quite so principled as much as it was quirky and fun.