I must be missing something. So Joe works as an employee at Somethingly earning 150k and won't jump ship to Yahoo for a pay rise. But after Yahoo acquires Somethingly -- and likely shuts down its product -- Joe will choose to keep working at Yahoo because...? Given that Joe presumably didn't have a say in the acquisition, what are the (non-monetary) merits of post-acquisition Yahoo over pre-acquisition Yahoo?
You're missing the effect of inertia. Most people do not continuously re-evaluate whether they would choose to be working at their employer if they were unemployed and seeking a new job. Instead, they continue the current status quo until things are blatantly wrong for them, and only then start looking around. This frees up mental energy to focus on things like actually getting their job done and having a life outside of work.
Yahoo is betting that it wouldn't be able to induce someone to jump ship, but once they have jumped ship, their work environment is "good enough" to keep them. Well, most of them.
It's the same reason why magazines and online services bill by subscription (it's a lot of effort to get someone to subscribe, but much easier to keep them subscribing), and why ISVs bundle a lot of crapware with major platforms (users wouldn't knowingly install their products, but if they're already installed they won't bother to remove them), and why UX designers make the default settings whatever benefits the company most (most users never change the defaults).