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If Nash had stayed in Dallas instead of going to Phoenix in 05 would they have won inn 06 and 11?

Yes, this is about whether it was Nash or the system in Phoenix that made him raise to being a much better player, particularly in the minds of many. If those Mavs teams do win both times, at least, couldn't be that he magically improved so much at age 30, so what would be the reasoning? Avery Johnson coaching? Nowitzki improving? What else could be used as the reasoning?

Dallas with prime Finley looked better built to win.

I think it’s fairly obvious that Steve Nash made the Mavericks offense substantially better. As an example, see the Mavericks’ offensive efficiency compared to league average each season in the minutes Dirk was on the floor in Dirk’s years as a star:

2001-2002: 109.7
2002-2003: 109.7
2003-2004: 110.7
2004-2005: 106.0
2005-2006: 107.3
2006-2007: 108.5
2007-2008: 106.2
2008-2009: 103.5
2009-2010: 102.6
2010-2011: 105.9
2011-2012: 102.1
2012-2013: 101.0
2013-2014: 105.2
2014-2015: 104.9
2015-2016: 103.0

All of those years with Nash were superior to all of the years without him. It seems fairly obvious that Nash improved the team offensively to a significant degree.

But, at the same time, despite the above numbers (which are genuinely extremely high in the Nash years) I don’t really think Nash and Dirk were a *great* fit offensively. Nash was a pick-and-roll maestro, but with Dirk that means pick-and-pop more than rolling to the basket, and pick and pop is not particularly efficient (even with Dirk), especially in an era where a large portion of those ended up being long-twos. So I think the issue Dallas had was that they had two great offensive players, but their offensive impact wasn’t maximized because they weren’t really a great fit, and meanwhile both were not very strong defenders. The result was still really good—they won a ton of games and only lost in the playoffs to really great teams—but I don’t think it was maximizing either player’s impact, and so they were probably better off on different teams. Whether Dallas still would’ve won in 2006 and 2011 is not a question we can know the answer to. It’s not at all implausible given how well the team played in the Nash years. And I also don’t think it should be limited to those years (maybe they would’ve won in a different year where Dallas without Nash didn’t do as well in reality!). But it’s perfectly plausible to me that both the Mavericks and Nash were better off being apart, so that they could be in teams that maximized their impact as much as possible.