Some commentators say that it is necessary for 12 tribes to be settled in Israel in order to keep shmita (rambam) but the fact is that two of the tribes had given up their shares in Israel to live on the other side of the Jordan river. The fact is that there never really were 12 tribes in biblical Israel and shmita was kept.
Another argument would be that there is a high likelyhood that those living in Israel have some ancestry from the 12 tribes.
Shouldnt shmita be kept deoraita by all in Israel?