If you start out being popular among people who prefer one approach to reporting and opinion, and unpopular among people who prefer something entirely different

If you start out being popular among people who prefer one approach to reporting and opinion, and unpopular among people who prefer something entirely different, and then you incrementally move away from what you used to do, at the same time as the second group of people have plenty of other options to choose from ... this is precisely what you'd expect. WaPo was viewed as a liberal, slightly left-of-center newspaper that did excellent investigative reporting. It therefore was all but ignored by those whose preferred version of the news is presented by e.g. FOX News. As it has taken some subtle and some not-so-subtle steps to move away from its reputation, that has done nothing to encourage FOX News viewers to read it, and a lot to discourage its former audience from continuing to do so. So one "side" has declined, and the other "side" has no reason to increase.