Siri had limited value as a standalone app interfacing only to public APIs and services, but huge value as a deeply integrated platform interface with links into all the platform services. The original company could never realise that sort of enhanced value, so as a standalone player it's value was limited and Apple could buy it (relatively) cheap.

PA Semi as a small independent chip designer could eke out a decent living doing small bespoke designs for companies here or there. That has value, but it's limited. However as the designers of the undisputed heavyweight champion mobile CPUs, in the most successful consumer product of all time, providing significant product differentiating performance and features, they have enormous value but _only_ if Apple buys them.

User ksec gave a great list of successful Apple acquisitions for which their core products still exist and are thriving and the above pattern applies just as well to all of them. That's the primary model for Apple. I'm sure there are cases where they buy teams for talent, but it's usually talent in a specific area Apple can directly leverage somewhere in their products.

EDIT: I would into be at all surprised if Apple went into these negotiations saying "We love your technology, it;s great. Here's an offer. BTW here's the brochure of another similar company with almost as good technology we could buy instead, integrate into our products and kill you with.".