The problem with Source is all these additions are made to an engine that was poor to start with. It's like trying to soup up a car from the 70s: sure, it'll run better than it did originally, but there's only so much you can do until the underlying chassis gives out.
Brush-based mapping cannot, in any way, replicate the amount of detail you can get out of a model. Plain and simple. Games are moving closer towards CGI-based films in terms of rendering prowess and detail, neither of which are in Source's favour.
Valve kept the format to keep things simple for them. Considering they hadn't shipped anything major since Half-Life, imagine if the team had to learn everything from scratch when it came time to start Half-Life 2. The advantage is that the rather large Goldsource modding community also had a nice head start when Source came along because they're, by and large, the same thing.
Valve's paying for their initial decision to stay in the brush-based realm. Nearly every engine that's come out since Source eclipses it in some way. Look at Uncharted 2 for the PS3: it's light years ahead of what Source can do, and that's an engine built specifically for a console. Look at CryEngine 2 as well: massive landscapes with an obscene amount of graphical detail.
People like Goldsource and Source because they stick with conventions that they're all used to. Goldsource modding is virtually identical to Quake and Source follows on from that too.
Besides, if anyone knows the most about this, it's Rabid: he's been wrestling with the darn engine for years.