I think you are overestimating Josh Smith's offense. Is he individually better than Singler? Yes, by a mile. But the team offense isn't as simply as that. Smith is very inefficient as a scorer (Kyle Singler actually had a superior TS%) because he can't shoot. In Atlanta that could be mitigated because Teague, Williams, and Horford all had solid-strong shooting skills outside the paint. In Detroit, he'll be playing alongside Monroe and Drummond a lot, players with zero shooting ability. Three non-shooters (and that's not even counting that Stuckey can't shoot and KCP hasn't hit much in summer league) on the court at once makes it VERY difficult to space the floor and get good looks.
Just look at teams like Memphis and Indiana. Both have superior offensive talent than the Pistons, but both scored only marginally better last year. Teams that can't shoot or pass struggle to score. Given we have a notable dearth of shooters (the ever-inconsistent Brandon Knight is the ONLY player currently on our team that shot about league average on jumpers), and we have no PG, it's highly likely we will struggle to score.
Teambuilding is not as simple as throwing talented players together. If it was, Sacramento wouldn't be such a wreck. The fact is, players have to actually fit together unless their talent is off the charts (like Lebron, young Wade, Durant, young Kobe). Josh Smith (nor Monroe or Drummond) is not at that level, and there is significant evidence to suggest he is not a good fit for the team. Atlanta only managed to be 15th in offense last year with Smith, despite Horford being better than Monroe and Teague being better than every guard we have.