This seems to invalidate the seven rabbinical laws (Sheva Mitzvot D’Rabbanan) introduced to face new problems.
In Rashi’s view the Torah is not denying the sages the right to interpret the commandments or introduce new subjects to the halachic agenda (Deut. 17:9-11), but telling them that they must not alter the established content of the laws, e.g. by having five passages in the t’fillin and not four, or five plants on Sukkot instead of four.
There is a further question suggested by the Torah passage, the apparent irrelevancy of verse 3: “Your eyes have seen that the Lord did concerning the worship of Ba’al Pe’or”. What does the Ba’al Pe’or incident have to do with not adding or subtracting? Is it because in that episode the people committed the two basic sins of idolatry and immorality?
The Alshich commentary says that the people thought they were improving on the teachings of the Torah, and so God had to punish them for believing themselves wiser than their Creator.