Length contraction, time dilation, and RoS are all consequences of the Lorentz transformations, which in turn are derived from Einstein's postulates, which includes the constancy of the speed of light. Therefore, if any
of these effects did not occur, Einstein's postulates would be violated.
Time dilation and Lorentz contraction lack the completeness of Einstein's postulates. They cannot be used as postulates to derive the RoS, constant speed of light, etc.