I would say that 20 years ago would have been a better time to stop endorsing.... and likely wont influence many/any.
I would also say FOX, MSNBC even NPR... I think most people already know what they are getting so whether they endorse anyone or not is even less important. Similar to oil and gas industry, teachers unions, teamsters etc. they usually have a position and pretty much know what you are getting.
I do think a reasoned/factual voice is important and not sure where you get that anymore. I would contend that if none of the R candidates over the past 40+ years has aligned with WAPO it might speak to the target audience but there is some level of bias.
From my end, I primarily get my news from WSJ but I realize (or at least assume) there is a R bias associated with the readership and the paper in general.
Thanks for your thoughts