Actually patents themselves don't have arbitrary length. They are 20 years and then done[1]. That said, there are patents on refinements or add-ons that come later, and so sometimes the patent expires but it has been replaced by a new way that is even better.
There is a scheme in the pharmacy market for shifting patents to prevent generics from being produced, that scheme would be outlawed if we had the right mix of legislators in Congress (basically fewer people who are dependent on these pharmaceutical companies for campaign funds and PAC support). But the original patents do expire, the trick is convince folks that they don't do what they do as well as the new thing does what it does.See here[2] if you want the boring details.
[1] I've got a bit over a dozen patents that arose out of work I've done for others and ended up becoming very familiar with them, even get the occasional expert witness work when trolls need to be disabused.