Jack was good enough in his prime (Indianapolis) to make the Spurs more of a big 4 and maybe the big 3 era would never have really existed. Jack wasn't less consistent than, say, Parker in '03. Yes, we needed to re-sign Manu, and that's why we didn't go out and grab Jason Kidd or Jermaine O'Neal that offseason. But we had more than plenty of cap space to bring back Jackson and Claxton. 3 years/9 mil even in those days was a bit insulting, the Spurs should have offered him a 1 year prove-it deal worth a lot more than what Atlanta offered and then made a decision on him and Manu both in '04.
Claxton's career went south pretty quickly after signing with a bad GSW team. He wasn't even the starting PG there IIRC. He did carve out a bit of a career with the Pelicans and Chris Paul for a couple years, but he would have had a much longer and more memorable career as Parker's backup for 10+ years instead of the career he ended up with. When you go to a garbage team, that's what typically happens. Jacque Vaughn was a prototype replacement but he didn't come until '06-07. Beno Udrih was kind of a bust. We didn't even have a backup PG in 2004 really. Claxton might not have been "the starter" on the Spurs but he was certain to get a lot of minutes here, especially whenever Parker got in Popovich's doghouse which was quite often back then.
And who's to say Parker and Claxton couldn't co-exist on the court sometimes? Especially in smaller lineups. Back in those days Parker shot more threes... Parker was really more of a small SG than a PG anyway, if ya think about it real hard. Who's to say that wouldn't have worked out? It worked out with Iverson and Eric Snow (flashy score-first small 2 guard + defense first all around solid PG) in Philly, and they made it to the Finals in '01 with that configuration without even having Tim Duncan or much scoring outside of Iverson for that matter.