Do you think Edge would have saved SD if he was there? Unified titles just suck, man.
The Smackdown Six put together in 2002 by Paul Heyman's request consisted of: Lesnar, Benoit, Edge, Mysterio, Guerrero and Angle.
Of those Six, the backbone of the brand was Edge and Lesnar, who were both less than the age of 30, at the time.
Once Lesnar gave his notice that he was leaving, Vince effectively gave up on Smackdown as a future alternative to Monday Night RAW.
Had Edge not gotten hurt, John Cena would have stepped into Lesnar's role but I also believe Cena eventually would have moved to RAW because SD is a 'wrestling based' show.
With Angle having neck problems of his own, Randy Orton may have had to switch to Smackdown and we know how long it took for Orton to get his wheels under him.
Edge couldn't have saved Smackdown by himself. Smackdown needed the name value of two elite stars, unless HHH were willing to move. Unlikely in my estimation.