There's a difference between being able to find whatever you look for, and being able to identify the most important thing to do right now. I think GTD is still current. GTD specifies principles and general methods, not a complete system; each user has to build their own. Electronic aids fit in nicely, for those who choose to use them. Paper is still excellent too, with advantages as well as disadvantages over electronic systems.
A person can only think about a small number of things at a time. You might be able to remember everything, but not all at once. A method of sorting and organizing is needed.
Inability is an abstract thing involving comparison with alternate universes; it cannot be experienced.