Agreed. If Corbett won the first Jeffries fight and retired, I would imagine Jeffires would have had a solid chance of becoming (in the context of this hypothetical) the 2nd heavyweight to regain the championship. And while I think this obviously elevates Corbett's standing significantly, if Jeffries did regain the throne and otherwise dominate, it may not have crushed his legacy.
Judging on how past his best O'Brien according to some primary sources outboxed Jack Johnson in 1909 ( Johnson's prime ) you have to figure Corbett if got in shape and had a shorter fight could make a good showing. Corbett was motivated, 33 and in top shape. His people think it was his best ring performance. The false information on this fight is it wasn't closed before the KO. Adam Pollack who wrote a book on both of them said it was any man's fight a few rounds before the KO, and a detailed round by round by the New York times ( Primary and local ), if you score the rounds by their call, has it about even with Corbett fading.