My supposition is that this is a response to changing tastes over time. However, I think 10 videos or less it probably an overreaction to this problem. For exa

My supposition is that this is a response to changing tastes over time. However, I think 10 videos or less it probably an overreaction to this problem. For example, let's say they have a user who watches 100 videos a week, for ease of math. 50 of those videos are in "core" areas of interest - these do not change over time. An additional 35 are in areas of secondary interest: topics which have piqued the viewer's curiosity, but not deeply interested them. We can expect these topics to change every [1,4] weeks. The remaining 15 are referrals or clickbait from other websites. How can YouTube differentiate between these three classes of videos? The first class will be heavily represented in their subscriptions. Presumably, the viewer will prefer their recommendations to ignore the third class (clickbait). The second class is the hardest, as the user may want these videos surfaced, but then want them to decay over time as their interests change. I think this is the problem that they are attempting to solve, with varying degrees of success.