AJ Styles was approached by WWE in 2002 but refused to re-locate to Ohio due to his family. CM Punk only caught people's attention in 2003 due to his feud with Raven in ROH. Likewise Samoa Joe was only just getting going in ROH and had previously been rejected by WWE.
It's unclear who went to the UK on tour but the following were on a break from WWE at the time: The Rock (would return in June), Shawn Michaels (June), Chris Benoit (June). Big ex-WCW names like Bill Goldberg and Scott Steiner who came in 2003 could be brought in earlier depending on when their Fusient contracts expire.
It doesn't look like the following appeared at Insurrextion unless they travelled there for other shows on tour: Hulk Hogan, Kurt Angle, Chris Jericho, Kevin Nash, Edge and Christian, Raven, Mark Henry, Rey Mysterio (not sure he had debuted yet), Billy Gunn, Chuck Palumbo, Rikishi, Lance Storm, Al Snow, D'Lo Brown, Test, Perry Saturn, The Hurricane, Billy Kidman, probably others I've missed too. Eric Bischoff may not have been there and could be called on by Shane and the Board to help run the company or at least the creative side.
There's also a wealth of talent recently debuted or shortly to debut: John Cena, Randy Orton, Batista, Sean O'Haire, Shelton Benjamin, Charlie Haas.
So clearly it would be a massive loss, but they'd retain a talented roster sufficient to continue programming and mainstream relevance. The roster was huge at the time, leading to the brand split, which would presumably be reversed.