They didn't get any worse. They were 12-22 with him and went 17-31 after dumping him, a virtual identical winning percentage. And then the next year, as we know, they had one of the biggest turnarounds in league history.
And yes, the Suns replaced him with Nash, the Nets replaced him with Kidd... but it wasn't like a marginal improvement, it was transformative. The Nets won 26 more games the next year, the Suns won 33 more.
Even the Wolves had a big bump in winning percentage when they replaced him with Terrell Brandon.
In a situation like "build your own" Suns team on a budget he should be worth an extra 10 bucks for other players. The dude was empty stats. He did have "talent" but it certainly didn't translate to his teams having any success, he ran a terrible offense and he hardly tried on D. He was an anchor.