Once in a blue moon you might get a team just blast their way through the season, but its extremely rare. When you look at this season, there's at least 3 teams good enough to compete for the title, so unless City manage to do something very unusual there will almost certainly come a point in the season where their form dips. At that point either they dig in and show grit, and find a way to keep collecting the points anyway, or they risk getting overtaken. If that happens, then its a hit to the confidence that's an extra challenge to overcome as they try and pull themselves back into form.
Few things are more demoralizing than watching your rivals playing like crap and still walking away with 3 points week after week. You guys used to do it all the time under SAF, and as a rival it was devastating. Mourinho ingrained the same mentality into Chelsea long ago, and at the times when we've had successful seasons, that's been a huge part of the reason why.
As for City leading, its 6 games into the season and you're level on points. It's hardly evidence of anything, especially considering City actually have less points now than they did after 6 games last season.