Adam Clarke on Hebrews 7:18

For there is verily a disannulling - There is a total abrogation, προαγουσης εντολης, of the former law, relative to the Levitical priesthood. See Heb 7:19.

For the weakness - It had no energy; it communicated none; it had no Spirit to minister; it required perfect obedience, but furnished no assistance to those who were under it.

And unprofitableness - No man was benefited by the mere observance of its precepts: it pardoned no sin, changed no heart, reformed no life; it found men dead in trespasses and sins, and it consigned them to eternal death. It was therefore weak in itself, and unprofitable to men.

The Jews, who still cleave to it, are a proof that it is both weak and unprofitable; for there is not a more miserable, distressed, and profligate class of men on the face of the earth.