Matthew 5:19 (relaxing commandments) is anti-Pharisee polemic, not a universal command to keep the Mosaic Law

The warning against relaxing commandments in Matthew 5:19 is directed at Jews, in a pre-New Covenant setting, and is specifically targeting Pharisaical distortion of the Law through added traditions. Jesus wants the Law kept in its full, undiluted form so that people will see they cannot meet its standard and thus need the fulfillment Jesus brings. The verse is not a timeless command to all people to obey the Mosaic Law.