These old covenant laws are obsolete, because the covenant itself is obsolete. The New Testament does not comment on the validity of these laws. It simply declares the covenant obsolete, and there is nothing in the new covenant that would cause us to conclude that the sabbatical and jubilee years are still required. These laws were given only to Israelites, only for the land of Canaan, only for the time period of the old covenant. Although we might expect that the law had agricultural benefits, the Bible does not make that claim.
Some farmland needs to be left fallow more often, and some less often.
The Bible does not give us authority to command these same customs for other people in other lands. Similarly, the Jubilee year had valuable economic results, but it was a civil law that Christians cannot require today. The old covenant required annual worship festivals. It specified the date and the place, the manner and the people to whom the commands applied. God did not command gentiles to keep this festival.