[Yo-Yo]

Yes, it would be horrible, and a way to pack the court.

I found the statute: https://www.law.cornell.edu/uscode/text/28/371

(a) Any justice or judge of the United States appointed to hold office during good behavior may retire from the office after attaining the age and meeting the service requirements, whether continuous or otherwise, of subsection (c) and shall, during the remainder of his lifetime, receive an annuity equal to the salary he was receiving at the time he retired.

...

(d) The President shall appoint, by and with the advice and consent of the Senate, a successor to a justice or judge who retires under this section.

The statute says "the President shall appoint ... a successor ... who retires ..." It doesn't say "the President shall appoint ... a successor ... who intends to retire ..."

I think the confirmation by the Senate of Ketanji Brown Jackson was premature according to the statutes, but it is what it is, I suppose.

[Jamestown1630]

To pack the Court, they’d have to change the law first.
It’s been codified at 9 justices since 1869.

[Elsie]

and shall, during the remainder of his lifetime, receive an annuity equal to the salary he was receiving at the time he retired.

Who; in their right mind; would continue 'working'???