My take on this would be that 64-bit version of the OS will have to do for 32-bit applications what 32-bit version of the OS did for 16-bit applications -- at least until there is a sufficient pool of 64-bit native applications (which probably won't happen for a while). Dropping support for legacy apps would prove to be fatal for a platform -- unless it enjoyed a monopoy, or at least have sufficient percentage of "loyalists".