Just an issue of timing and a bit of an anomaly really. Nixon got 4 picks in his first term alone and then Ford got a fifth pick for that GOP administration and Marshall was appointed at the end of the Johnson/Kennedy admin meaning there was only a possibility of 3 seats to pick from, and of those 3 Brennan was the oldest at 70 at the start of the term. If Carter had been re-elected Brennan may have retired in 83 or 84. Potter Stewart and White were especially young for Scotus, as they along with Thomas are the only people appointed to the court under 45 after the second world war.
We could be in for a similarly weird situation with Biden as he could very well serve for 2 terms and only get 1 nomination or some variation of it. Sotomayor could end up "trapped" on the court like Marshall and Brennan in some respects, particularly if a dem wins in 2024. The dems will probably not have the senate making it a possibility that no nominee leaning left could be confirmed for her and she would be 75 when the 2028 election rolls around. If the GOP nominee were to win 2 terms from there then she would have to go until she is 82/83 like Breyer now and supposedly her health is not the best.
The next GOP president is practically guaranteed 2 picks as I assume Alito will step down if the GOP wins in 2024 as he will be just turning 75 and he does not strike me as a guy who wants to be there for a long time and I assume Thomas will go after 33 years, and if not then Roberts will leave.
Thomas seems to be determined to become the longest-serving Associate Justice in the Court's history, and I could very well see him remaining on the bench until he dies. But I'd agree with you about Sotomayor, and I think it's possible that Roberts might retire under the next Republican President. Interestingly enough, every Chief Justice over the past 68 years has been appointed by a Republican President. The last Chief Justice to be appointed by a Democratic President (Fred M. Vinson), served from 1946-1953. Lyndon Johnson had the opportunity to replace Earl Warren in 1968, but he botched it by choosing the ethically dubious and controversial Abe Fortas.
Had Johnson nominated someone less controversial, then the legacy of the Warren Court would have been cemented further, and it would have had a
major impact on the Court's direction. A Johnson-appointed Chief Justice-assuming he had chosen someone in their early 50s who served for ~20-30 years-would probably have meant that neither Burger nor Rehnquist would have ever obtained the position.