That's true, but we need to get it really, really close. Even relatively small statistical deviations from the behavior of the real neurons are probably intolerable. Besides, real neurons are not interchangeable: they have unique statistical biases and are influenced by a variety of factors not modeled by modern simulations, like neurotransmitter diffusion, glial activity, and subtle quirks of specific dendrites and axons.

Right now, even if you gave us a high-speed brain scanner, a high-speed computer, and an unlimited budget, we wouldn't have the capability to interpret the image data the scanner produced, or even be quite sure which immunostains to use for the optical imaging to pin down the required details. I expect it to take at least five to ten years for us to get the theoretical details ironed out.