Nice list.
Both 48 years and 87 years ago, only sitting presidents Richard Nixon and Herbert Hoover were alive. However, in 1933, FDR was already president-elect after Calvin Coolidge died on January 5. It's also interesting that both presidents who died before, Johnson and Coolidge, ended their presidency in a year that ended with 9 after roughly one and a half term and died in January four years after leaving office.
Three times during the 20th century, there was a period in which there were no living ex-Presidents: June 1908-March 1909 (Grover Cleveland's death to William H. Taft's inauguration), January-March 1933 (Calvin Coolidge's death to Franklin D. Roosevelt's first inauguration), and January 1973-August 1974 (Lyndon B. Johnson's death to Richard Nixon's resignation). Johnson died just two days after Nixon's second inauguration, and on the same day that Roe v. Wade was handed down. Had he won reelection in 1968 and died at the same time as he did in OTL, Johnson would have had a very short presidential retirement. But then again, given the perilous state of his health, he might very well have died sooner and while still in office in such a scenario.
Of course, his health was only so perilous at the end because he picked up smoking again almost immediately after he left office (& I say almost immediately in the literal sense: it was on the AF1 flight from DC back home to TX). Had he been re-elected & continued to receive the best medical care on the face of the Earth, who knows what would've happened?
Your arguments are persuasive, it's reasonable to believe he'd live longer if he stayed in office sooner...
