Breyer announced his intention to retire at the end of the session, assuming that his replacement had been nominated and confirmed. Implying that if a successor was not appointed and confirmed, he was not going to step down at the end of the session.
One retirement announcement was definite, and one was contingent.
If the President can nominate, and the Senate confirm, a successor prior to there being a vacancy, then what would stop the President from nominating, and the Senate confirming, successors to all of the Justices?
There would have to be an indication of intention to retire. It wouldn’t fly otherwise, especially as the Constitution is silent on the issue.