Culture is set by the execs, and their senior managers, not the developers. Buying up new companies won't change the culture unless they are integrated into a new management structure. Sure, a good junior or mid-level manager can maintain a strong culture in their team/organization. BUT only with senior exec support - otherwise they will fall under corporate oversight for everything - from facilities policies (free snacks, beer, soft drinks, breakfasts? huge desks? breakout rooms?), HR processes (hiring, performance, salary increases, role changes), IT processes (hardware/software freedom), etc. There are talented managers who can present a view of full cooperation, but still do their own things - though again, to pull that off, they usually have some senior exec support. The more contact with the rest of the org. the more its culture seeps in.

Just think about it in terms of code - you write some code as a standalone demo - to bring it into your main application, you end up having to integrate into most of the existing framework, and that influences the original demo code - often for the better, but that depends on the quality of the existing code base - and how well it was architected (by the code equivalent of the execs).

Very few companies do acquisitions where they carefully protect the acquired company. The assumption is that the buying companies culture is better! One company that does do this quite well is Conde Naste. (Wired, Reddit, etc. all have quite separate identities, and from the outside appear to have autonomy.)

When you start to see an exodus of long term Yahoo! execs, then she's probably doing something right. Whether Wall Street would reward her for that, is a totally different question!!!