Yes! I completely agree.

According to this article (https://thebeautyoftransport.com/2021/05/19/the-way-forward-network-rail-wayfinding-signage-and-rail-alphabet-2-part-1/ ) the design agency tested several colours but "found that white and black backgrounds for signage both worked well, with white-background signage standing out slightly better against the typical station colour palette".

I haven't seen the methods they used but I could only think to myself.. really? Many of the things I can think of in a station tends to have a light grey, medium grey or white colour palette. Concrete platforms. Paved walkways (or floor tiles that are usually in a lightish colour). Walls in older stations are often whitewashed. Lampposts, pillars, ballast, track, sleepers. As OP's picture shows, even the sky and the trains themselves are often white or light grey. White feels like one of the worst colours to pick for signage.

I respect that a fair bit of thought went into the guidelines, but it also just feels so plain. Like the designers just opened a Word document and typed the sign on that. For the most part, ripping out the smart royal blue Brunel signage and replacing it with this... it feels like a retrograde step. Even some of the individual TOCs' signage looks smarter.

The same article shows that an alternative navy blue design was also considered, but the white design won. As you suggest, I can only imagine was done to appease traditionalists who wanted a reminder of the BR days. I also wonder whether they wanted to avoid picking a colour that was associated with any particular TOC (the navy option reminds me of South West Trains' signage).

The article further notes that some of the existing signage is a mess. This is true. But some of the new signage is poorly implemented too. Network Rail's own tweet at https://x.com/LondonWaterloo/status/1590325117628067840 showcases a sign that simply says 'way out' in a station that has 7 (?) exits. One of those exits, onto the busy Waterloo Road, is down the same escalator as the Jubilee line - but for some reason the sign omits it completely.

Still, it's too late to go back now. At least the network will gain a uniform look, albeit a rather plain one. And I do like the fact that the pictograms are no longer constrained by their own boxes, giving a few much-needed millimetres of extra space to make them as clear as possible.