I suppose we are probably talking about slightly different things, which to be fair is probably my fault. I was really responding to the comments on its design, rather than how it feels today.
I do completely agree that it will probably turn out that drastic measures now will be the only way to make it feel pleasant. I guess my main point really was that this was not how it was designed to be. Indeed I think the reason the natural light is mostly (if not pretty much all) in the concourse/holding area is that this section of the station was always meant to give a feeling of being the section used as a departure lounge and destination, the focal point, and I don't think anyone ever anticipated precisely how many people would need to queue on/off the platforms, at the barriers or run around with 2mins to go in quite an enclosed environment. Because of the cluttered atmosphere on the concourse, I think the apathy has spread to maintaining a pleasant atmosphere on the ramps and on the platforms, too, which is very unfortunate. Even things like the tone of the lighting and the way you approach basic facilities such as the Tube station entrances don't feel right. These things could have been maintained in a better way. They weren't.
I wouldn't ever claim Euston is "beautiful", as such - that's not how it was meant to be - but it could have been kept a lot more striking, interesting and worthy of destination status.
As for the comment about being a bit of a warren of corridors - I completely get that, and why it can feel awkward (personally I get a bit of excitement about secret corridors and mothballed industrial facilities, but then again I'm a bit weird like that). However, most large or medium-sized stations have hidden areas, disused rooms and that sort of thing. It's not new, and indeed even in older and more pleasant locations, it's not necessarily any better. In fact, to be honest, even some of the smallest stations on my routes have their secret cubby-holes and although it can be fun for me to find out a bit more about them, frankly it doesn't mean they'd actually look or feel any better than Euston.