Ann: HGST Renews for 9th Consecutive Year, page-27

Companies piece together IP from all over the place to make their products work. They have to pay the owners of each patent a fee accordingly. WBT has licensed IP from third parties to make their RERAM work.

You can develop more IP on top of the original IP, but you can only license/sell the IP that you own. Any third party you sell your IP also has to strike a deal with the original IP owner if they want access to everything.

Of course, once the 20 years is up, you can do what you like with the expired IP. That's how AMD came into being. They took the original IP that Intel owned for X86 processors and started making their own CPUs with it. Initially they were considered to be lower quality knock-offs of Intel but now arguably AMD fabs the superior products (Intel still has the brand cred)

It's also how the solar panel industry took off - all the original patents (that were developed in Australia btw) are so old manufacturers don't have to pay fees to use them.