the fact we cant keep the repo in our working memory is a flaw of our brains. i cant see how you could possibly make the argument that if you were somehow able to keep the entire codebase in your head that it would be a disadvantage.

Information tradeoff. Even if you could keep the entire code base in memory, if something else has to be left out of memory, then you have to consider the value of an abstraction verses whatever other information is lost. Abstractions also apply to the business domain and works the same.

You also have time tradeoffs. Like time to access memory and time to process that memory to achieve some outcome.

There is also quality. If you can keep the entire code base in memory but with some chance of confusion, while abstractions will allow less chance of confusion, then the tradeoff of abstractions might be worth it still.

Even if we assume a memory that has no limits, can access and process all information at constant speed, and no quality loss, there is still communication limitations to worry about. Energy consumption is yet another.