TDM is not a replacement. The only way we could do that is if we were to somehow track down every single copy of the game ever produced and strike it from existence completely. Then remove every single copy from every hard drive. Then we would need to destroy every fan mission and every backup in the world. Then we would have to erase Thief from the memory of every Thief fan and then....well, then nobody would know what Thief was, nor would they likely care what TDM was. TDM couldn't have existed without Thief.
It's an extension, not a replacement.