Agreed, Network Rail lifted the appearance of Euston's concourse very effectively, made a great clean space of it and used lighting to make the most of its austerity. Then people must have moved on, and a new lot didn't seem to see what they had. (That might be a terrible take though as I was only a very occasional user of the station, people who passed through it every day may have felt differently.)

Mark

 
This was within my lifetime, another fact that makes me feel very old ! I do not actually remember it, being too young to be interested in such things.

I DO remember a letter in a newspaper, calling for a slight reduction in train fares between Euston and the North, on the grounds that the new Euston station was furthur north, and distance travelled by the train therefore was reduced !

I rather like the new Euston station. The appearance of the concourse was somewhat spoiled when the lighting was changed from the previously used cool white metal halide lamps to orange high pressure sodium lamps. This made the marble floor look black and dirty, whereas the previous lighting showed of the green colouration to advantage.

 
From https://heritagecalling.com/2014/12/15/9-lost-railway-stations/



And from the public domain, the Great Hall and the 1888 track plan