Branding, or at least a distinctive house style, is important not just for commercial reasons. It's important for expressing the corporate identity of the organisation. Hopefully we are getting away from competing and clashing imagery and back towards a recognisable visible 'look' for the national rail network (lowercase until we decide what to call it; I'm not a fan of 'Great British' anything - too Brexity for me).
Simplicity is good, but not banality or such minimalist styling that the signage could be anywhere or for anything. London Transport (aka the LPTB or TfL) was the pioneer of this in the 1930s and it has not completely lost its touch. The roundel and the Johnston typeface make it obvious where you are and which bits of signage are important to you as a passenger. The BR logo and Rail Alphabet have served the same purpose for the national network. Updating this is fine, but tinkering with it arbitrarily, and confusing matters by imposing local or TOC-only variations, is not helpful. Even after several decades of privatisation, most people recognise the former and are confused by the latter (especially if the train they are travelling on is branded differently from the station they are travelling from.)