The Constitution imposes a duty on the President to take care that the laws of the United States be "faithfully executed" (Article II, Section 3, clause 5), so the President has both the power and the Constitutional mandate to make sure that The Judiciary Act of 1869 be followed and that the SCOTUS be made up of the correct number of Justices as mandated by the law.
The Constitution is the SUPREME law of the land and the HIGHEST law of the land. The Judiciary Act has been changed several times; it is NOT an amendment to the Constitution. The President has an obligation to faithfully execute laws; including the Constitution!!
Article II of the Constitution requires the President of the United States to nominate Supreme Court Justices and, with Senate confirmation, requires Justices to be appointed.
This was for the division of power between the President and Senate by the founders, Article Two of the United States Constitution requires the President of the United States to nominate Supreme Court Justices and,
with Senate confirmation, requires Justices to be appointed. This was for the division of power between the President and Senate by the founders, who wrote:
"he shall nominate, and by
and with the Advice and Consent of the Senate, shall appoint ... Judges of the supreme Court..."
The POTUS only has the power to appoint WITH the consent of the Senate. The Senate is not mandated to vote on an appointment. Likewise, the POTUS can also withdraw his appointment.
https://en.wikipedia.org/wiki/Appointment_and_confirmation_to_the_Supreme_Court_of_the_United_States