Nice way to move the goal posts. You said He unsuggested it after I walked through the umpteen problems involved in implementing this over hundreds of millions of accounts. which has nothing to do with how much money Apple makes, or what fraction of a fraction of a fraction of a percent of Apple's income it would take to implement and maintain, or what kind of support problems there would be. Considering how Apple already doesn't do anything with the top 10 lists, they could continue to ignore a new implementation and have the same set of support problems/costs. There's nothing about "IT MUST BE CORRECT", in fact making it customized per user means there is no single "correct" top 10 list to show.

You're welcome to start listing the actual umpteen problems that a billion dollar company would incur rather than just talking out of your ass about "Apple's billions" to a bunch of people, myself included, who have most likely implemented similar personalization systems at countless other sites using a lot less human, financial, and processing capital than Apple has. Apple "wastes" a megabyte for a picture of a wave on their current home page. Any change here is likely to move the needle in a positive direction more than the increased sales from having a massive picture of a wave on their home page.

Every engineering problem involves the "how much effort vs how much profit" calculation. In fact, in a sane environment, it's the first problem you solve before you start thinking about intersecting millions of lists.