The things is the brain isn’t a computer, and it doesn’t work like one. Computers store information as a series of bits, literally as the presence or absence of information. Thus when a computer stores a memory of a face for example it literally memorises what parts of the face contained nose, and what parts didn’t, so a person with a large nose will simply have more information in the ‘nose’ part of the picture and less in the ‘cheek’ part pf the picture. That sort of processing works well for retaining very fine detail but it is hard to compress. It is also wasteful of space because in order to memorise a person the computer needs to memorise the face as a whole, the body as a whole, specific identifying markings at an appropriate level of detail. Then it needs to store different pictures of the person when happy, when sad and so on.
In contrast the human brain, as far as it is understood, stores information as reference information. So when a person starts life they memorise a specific face, presumably their mother’s. That initial memorisation is probably just as memory intensive as the way computer does it, relatively. But the efficiency of the brain is that every other face is stored as a reference to that. So Dad is stored as Mom with a narrower face and a beard, that’s all the information needed to distinguish those faces. Then Uncle Dave is Dad without a beard and with glasses. And the babysitter is Uncle Dave without the wrinkles, and with a longer nose. And that goes on throughout life.
The beauty of that system is that, in theory, the memory capacity of the brain is nearly infinite. Every new piece of information is referenced to previously memorized information, meaning that as the amount of information increases the amount needed for the next piece is reduced in proportion. The first face might take 10, 0000 ‘bits’, but the second face will only take 500 bits because it is just a header file referring to the first face and with a simple instruction to narrow the first face and add a beard. The next face might only take 100 bits because it’s just the first face + the second + instructions to subtract 20 years, and so forth. By the time a person knows 100 faces they can probably store the whole of humanity with header files saying “Person7s chin, person 8’s ears, person 179’s smile” and so forth. No new information is being stored per se, it is just a matter of cross-referencing existing information.
Now all those header and reference files will take up some space, so the brain will become full eventually, but it so over engineered that nobody has ever lived long enough to experience anything approaching it. People of 120 can still remember what their parents looked like, can draw pictures of their school and so on. Memories fade because of disuse, not because the brain runs out of memory capacity. As such it’s impossible to know what might happen if the brain ever reached capacity.