Counterpoint: only Republican Presidents being allowed to appoint SCOTUS Justices leads to them being more conservative than the country in general.
Neither a president's nor a justice's party affiliation is any assurance of how they will rule once they're seated on the Court.
No justice's tenure on the Court more conclusively epitomizes this verity than that of Chief Justice Earl Warren.
"The Warren Court has been recognized by many to have created a liberal 'Constitutional Revolution.'"
Chief Justice Earl Warren was nominated by Republican President Eisenhower who later declared Warren's nomination was “the biggest damn-fool mistake I ever made."
Wikipedia
Warren was born in 1891 in Los Angeles and was raised in Bakersfield, California. After graduating from the University of California, Berkeley School of Law, he began a legal career in Oakland. He was hired as a deputy district attorney for Alameda County in 1920 and was appointed district attorney in 1925. He emerged as a leader of the state Republican Party and won election as the Attorney General of California in 1938
Earl Warren...served as the Republican Governor of California from 1943 to 1953 and Chief Justice of the United States from 1953 to 1969.
Warren is generally considered to be one of the most influential U.S. Supreme Court justices and political leaders in the history of the United States.
The Warren Court has been recognized by many to have created a liberal "Constitutional Revolution", which embodied a deep belief in equal justice, freedom, democracy and human rights.
In July 1974 after Warren died, Los Angeles Times commented that "Mr. Warren ranked with John Marshall and Roger Taney as one of the three most important chief justices in the nation’s history."
In December 2006, The Atlantic cited Earl Warren as the 29th most influential person in the history of the United States and the second most influential Chief Justice, after John Marshall.
In September 2018, The Economist named Warren as "the 20th century’s most consequential American jurist" and one of "the 20th century's greatest liberal jurists".