That cast was not weak for 2003. All the contenders out west was dealing with injuries to their superstars. Both Dirk and Webber went out in the playoffs that year.
Spurs were already up 2-1 on the Mavs before Dirk went out. Dirk had 28yo Nash, 29yo Finley, 31yo Van Exel, 26yo LaFrentz, 26yo Bell, 26yo Najera, and 32yo Walt Williams. That is a better supporting cast than 20yo TP shooting 40%, Stephen Jackson, Bruce Bowen, and rookie Manu shooting 40%.
Shaq had 24yo Kobe as his #2 instead of 20yo Tony Parker. The rest of that cast (prime Fisher, prime Horry, Devean George, Medvedenko, and Shaw) was good enough that the Lakers preseason odds were +140 compared to +400 for the Spurs, despite Duncan having won MVP the previous year.
For the playoffs, Duncan averaged 25-15-5 and 3 blocks a game on 53% shooting. He led the team in all four of those categories (points, rebounds, assists, and blocks) with no one else even close. 10 more ppg than anyone else, 9 more rpg than anyone else, 2 more apg and 2 more bpg than anyone else.
Against the Mavs, 28-16-6 and 3 blocks/game on 57% shooting.
Against the Lakers, 28-12-5 and a block a game on 53% shooting.
Against the Nets, 24-17-5 and 5 blocks/game on 50% shooting.
He had to be the dominant scorer in every series. He had to be the main distributor in every series. He had to be the dominant rebounder in every series. He had to be the dominant defender in every series.
And it's not like he was stat-padding in easy series and blowouts. He HAD to play that well for the Spurs to win in every series.
Sorry, but that's a carry job. Duncan's supporting cast wasn't trash, they were good enough to win a title. I'd say they were comparable to the Nets supporting cast (but less experienced), and Kidd could never win a title with that cast. Duncan could because he straight carried that team on both ends.