Let's say that I invent and patent something, a computer chip for example. Then over the next 15 years I add a whole bunch of new features to that invention and patent each of the modified designs. Now my chip runs faster, handles new functions, etc. All of these modifications expand the chip's abilities but are useless without the underlying chip's original design.
So what happens to my rights in the original design -- which is central and essential to the new designs -- when the original patent expires? Does that mean that as soon as the original patent expires everyone who wants to can start using my old design in unlimited ways without licensing (except of course for any uses that would run afoul of duplicating my newer devices)? Could they manufacture and sell chips identical to my first design even though that design is the core integrated into my newer designs? Would I have any rights to sue them for infringement of any kind?