I agree that's a possible reason. But I don't think it's specifically necessary for Twitter, especially the sort of power user who wants a chronological timelin

I agree that's a possible reason. But I don't think it's specifically necessary for Twitter, especially the sort of power user who wants a chronological timeline. Whether or not they show topic data for a particular tweet, they generally have it. I only use the chronological timeline, and I cleared out the interests list a year or two back, but they have built up a profile of hundreds of interests for me. I suspect forcing the ranked timeline on people is all about a small portion of Twitter juicing their engagement metrics, with no grand cross-team strategy in play.