Correct. Adding 32-bit clients for two operating systems costs a lot (particularly in terms of testing), and simply won't matter very much. LOTRO isn't CPU-bound in nearly any situation.
Requiring DX10/DX11 might be feasible, since only about 3% of people on Steam are running DX9 any more. DX11 and Windows 7/8, as the other poster was suggesting? No way. 38% of the gamers on Steam are missing one or the other (or both). When that number falls to more like 3.8%, they might consider it.