inertia keeps the earth and moon system as it is until an external force acts on it (say a collision)
so there would two scenarios:
#1: Where there would be something else replacing inertia, which would decide of what would happen in the above said situation...and it will lead to a different set of laws of motion
#2: there won’t be a replacement, no laws, which wrecking havoc in the universe...and probably throw in the possibility of a existence of life into question