The last sentence I meant more generally than the domain of cache algorithms, setting up "ideal" as a contrast to "pragmatic", since I'm taking flak for being "academic" (haha, you should have seen my grades). One lesson I've learned in my career is that there are instances where it pays to take some time, see what the research says, experiment, and then refactor. It takes a lot of time, but the best software is also long-lived so it can pay to take the time to make one's software the best. antirez has 24 bits to play with and doesn't want to make major changes, probably also doesn't have a lot of spare time, and that's his choice.
You still don't understand what Redis is and what antirez wrote in first reply to you. Your "major changes" will have impact on my use cases. Redis is not only used for caching, I am not using Redis in production for caching and many other people also do not use Redis for caching.
Have to retract the last comment['s promise of final note] and (academically) note here that "ideal" and "optimal" are distinct notions in computer science.
A provably "optimal" algorithm is implementable. An "ideal" that requires magical facilities is a trope to make a point and spare one from attempting the impossible.
But I am also aware of the oracle algorithm.