I don‘t think human memory works like that, at least not in theory. Storage is not the limiting factor of human memory, but rather retention. It takes time and effort to retain new information. In the past you spent some time and effort to memorize the shopping list and the phone number. Mulling it over in your mind (or out loud), repeated recalls, exposure, even mnemonic tricks like rhymes, alliterations, connecting with pictures, stories, etc. if what you had to remember was something more complicated/extensive/important. And retention is not forever, unless you repeat it, you will loose it. And you only have so much time for repetition and recall, so inevitably, there will be memories which won‘t be repeated, and can’t be recalled.

So when you started using technology to offload your memory, what you gained was the time and effort you previously spent encoding these things into your memory.

I think there is a fundamental difference though between phone book apps and LLMs. Loosing the ability to remember a phone number is not as severe as loosing the ability to form a coherent argument, or to look through sources, or for a programmer to work through logic, to abstract complex logic into simpler chunks. If a scholar looses the skill to look through sources, and a programmer looses the ability to abstract complex logic, they are loosing a fundamental part of their needed to do their jobs. This is like if a stage actor looses the ability to memorize the script, instead relying on a tape-recorder when they are on stage.

Now if a stage actor losses the ability to memorize the script, they will soon be out of a job, but I fear in the software industry (and academia) we are not so lucky. I suspect we will see a lot of people actually taking that tape recorder on stage, and continue to do their work as if nothing is more normal. And the drop in quality will predictably follow.