Yes, he could have beaten Ruiz, Rahman and Peter. Those were his fights to loose. He was very selfish in loosing those fights, because he denied me a valuable propaganda weapon.
Dempsey could not have achieved much more in terms of contemporary glory, he pretty much set the benchmark for that. In terms of anoraks like us, yes there are a couple of key fights that might have made a big difference. It has to be noted, that Dempsey fell short because he had more glory than he could ever want in the wildest dreams of avarice, while Toney fell short without ever realy touching it.
No, Dempsey fell short because he went to great lengths to keep the glory, those lengths being a minimization of risk while maximizing reward. To be more specific, he sat on his coddled ass and almost exclusively fought no hopers with his house ref during what is called his "reign".