einstein didnt state that c is constant, rather he based his theories on the idea that c is a constant, which has since been conclusively proven.
note that for the michaelson-morely experiment, they were trying to prove that the aether existed, and that they rotated the apparatus through ninety degrees t determine if the orientation made any difference to the results.
my teacher used the analogy of swimmers in a river, one swimming 1 km up the river and back, the other swimming 1km across the river and back, who would be back first if they can both swim at 4m.s-1 and the river current is 2m.s-1