The notion that these guys get life appointments has ALWAYS been one of the most idiotic decisions that the Founders made.
In point of fact there is nothing in the Constitution that actually says the appointment of a Justice is "lifetime." The Federal Judiciary is described under Article 3 of the Constitution. All it says is:
"The judges, both of the supreme and inferior courts, shall hold their offices during good behaviour." The presumption it is a life appointment comes from the fact that there was no stated term limit in Article 3, unlike Articles I and 2 which give terms of 4 years for a President between elections, 6 years for Senators, and 2 years for Congressmen between elections.
Subsequently the only change has been to limit the President to no more than 2 terms of 4 years each under the 22 Amendment passed in 1947.
One President able to stack a court in their favor just wipes out the other side for decades... STEW - PIT
This concern presupposes that such appointments have always been political in nature, i.e. designed to push a specific political agenda.
But that was not the concern at the time the Constitution was promulgated. That developed over time as political parties became further and further apart ideologically, and then concerns about how appointments made based solely on "political views" became factually apparent.
That would be exemplified by "Democrat" appointees to SCOTUS who were Southern "slave state" proponents that remained on the Court after the Civil War. They were responsible for decisions like Plessy v. Ferguson, 163 U.S. 537, (1896).
Later, issues of "Court Stacking" were threatened by politicians, most notoriously President Franklin Roosevelt who wanted to appoint more Justices so that his political agenda would not be "overruled" by the Court.
This problem of Court stacking still remains, and is still typically pushed by Democrats for political reasons.
I do for many reasons. Not the least of which is that the SCOTUS should NOT be considered another "political football" where one political Party of another seeks to appoint persons solely because of partisan reasons.
However, I will leave it to others to flesh out the arguments for supporting lifetime appointments. There are many pro and con.