As pointed out by many others, there are several multiplayer games on the 360 that handle large amounts of data and manage to look good at the same time. And I've yet to see any developer say they had to make sacrifices because someone might be trying to play on dialup.
Those multiplayer games make their own sacrifices, especially in the areas of technical reliability (Halo: Reach, for example) and not being almost the exactly the same after each iteration (Call of Duty, for example).
Reach is a particularly good example of games that make alternate sacrifices (and it's the example I am most familiar with). Reach has better graphics and networking than Brink. However, those come with some pretty heavy sacrifices. The netcode, for example, doesn't sync nearly as much information as Brink does -- you can actually goad it into desyncing key objectives! And that amazing graphics engine? It's actually ridiculously unstable. Dropped frames are insanely common on custom maps, and are even a frequent occurrence
on built-in non-Forged maps and
on official, Bungie-made custom maps. Reach also sacrificed data storage reliability; it very frequently corrupts hard drive data (yes, any data at all that gets saved to the drive) beyond
all possibility of recovery. Their file system is as buggy as the Resistance is malnourished.
So that's an example of one of the "several multiplayer games on the 360" that handles tons of data and looks good, but it makes its own sacrifices. All things considered, Brink's sacrifices are extremely reasonable -- and in several cases, they're actually preferable to those made in bigger games.
[/essay] [/semi-rant]
EDIT: And as for developers talking about sacrifices... Why would they ever admit to sacrifices? They have a game to sell!