This could also mean that their coverage is getting less accurate overall. It's not a one-dimensional space (left/right), more like a multidimentional (left/right,accurate/inaccurate,...) space. It serves nobody to be in the "centrist/inaccurate" spot on that graph.

BBC4 documentaries and things like Arena remain amazing, so they can do quality if they want to and it's not overwhelmingly a cost matter. It's more like populism vs Reithianism, and if the BBC isn't Reithian it will end up not existing.