This is a math exercise, it assumes an infinitely long lived person, as well as a lot of simplifications. That's the same kind of arguments that can make sailboats go infinitely fast as long as there is any kind of wind.
It is not useless. It tells you essentially two things. First, it shows the limits of your model. If you have proven that sailboats can go infinitely fast, which is obviously not the case in reality, then it is an indication that your model is incomplete and you have to refine it until it stops giving out nonsense, or establish its limits.
Second, it shows that what you have taken into account in your model is not what limits the speed of sailboats or memory recall.
It's not a math exercise. The author poses a question that is in the realm of psychology or neuropsychology or something like that, and then jumps straight into solving it with pure math, picking premises out from thin air.
The premises are obviously wrong, like assuming an infinitely long lived person, which means the answers arrived at will have no correlation with anything factual. You can't fix that with any sort of refinement. To be refinable would require it to be coarsely correct to begin with.
It is not useless. It tells you essentially two things. First, it shows the limits of your model. If you have proven that sailboats can go infinitely fast, which is obviously not the case in reality, then it is an indication that your model is incomplete and you have to refine it until it stops giving out nonsense, or establish its limits. Second, it shows that what you have taken into account in your model is not what limits the speed of sailboats or memory recall.