A simplifying assumption of many journalists, consultants and academics about the retention practices of law departments is that a law firm liked by a department can take on more work. Those lawyers who retain law firms know that the world is much more complex.
The infinite-capacity assumption does not, in fact, pan out. Larger firms do have more pent up supply than do smaller firms, but even they (generally) acknowledge limits on how many hours they can squeeze from associates.
More to the point, once you penetrate to specialty practices – such as European antitrust litigation or nanotech patent applications – the infinite-capacity assumption becomes ludicrous. Very finite capacity then joins with something closer to infinite fees.