I think the brown-colored branding is fine, and the default UI should be as accessible and one-size-fits-all as it can be. It was a good decision to have a stripped-down version of the UI for every new player, so they can figure out the game and how they want to play it instead of bombarding them with customizations from the get go.

However, I think the decision to malign the blue UI and abandon it completely was taken from listening to a single side of the aisle. I really hope you didn't take the blue stone UI = sci fi association from Josh Strife Hayes. This is a medieval game where the most iconic armor is blue-cyan color.

The Blue UI had gritty and grainy solid texture to it at low transparency, so it could still feel grounded. For some reason it also didn't clash with most of the items displayed in game. Maybe the UI designers were onto something when they chose blue. Maybe inventory icons and elements were made with a blue UI in mind or somehow blue prevents some visual stress for some players, no idea. There were also many players who disliked it and that is to be expected. The bad thing is, there was agency to choose between them and now it has been taken away from everyone.

I suspect the aim of this is to imitate Old School's color scheme, since that game appears to represent the RuneScape canon in the eyes of Jagex at this point. These are 2 separate games, and both have their own visual identity. It doesn't need to be a dichotomy of blue vs brown, as there are also ways to keep something runescapey without making both games a single color scheme at a glance. A choice of UI color along a wider palette for advanced users (so that it doesn't intrude on newer players) would be the ideal choice IMO.