I agree, and I disagree.
Take Chelsea for example. They proved that you can buy trophies. Chelsea, arguably had a much better team to start with, and a much more talented manger.
I don't know about Manchester City, they still have a fairly average defence. The better teams in the League (the ones they're up against) have much better defenses and could probably exploit City's weak back line.
Chelsea had a top 2 team and strengthened the team(without Mourinho's help) and Arsenal were much worse the following year rather than Chelsea improving dramatically under Mourinho, the majority of the winning players were already there.
People are discounting City because, well, not entirely sure why. I don't think Ade and Tevez are great players, far from it, but both will get goals, having Tevez as a first team starter didn't stop Utd winning the Champs league and the title, and another title after that. He wasn't the guy that made the team click but he wasn't so bad he prevented it either.
People like to rag on their defence, which started off terribly, but that was mostly Ball/Hamann/Zabeleta who between them were simply a joke. all 3 are done now, Onoura was also terrible and was only playing due to Kompany being injured. Dunne and Richards for 2 years before were great, and dunne for another couple years before that.
They'll pull it together with a better squad around them and without such terrible players playing anymore.
Remember Hamann/Vassell played half a season last year. Robinho was quality, he wasn't poor in away games he simply couldn't do much about those other few who were terrible, who won't be there. Ireland isn't quite Fabregas's level, but he was still superb all last season, again let down by those around him.
Not saying they'll certainly finish in the top 4, but they have a serious chance, as do the normal top 4. It took Chelsea years to get up to their level and City need time, more importantly their owners know this even if the fans don't.
They also have maybe the best defender in the league behind them and now one of the better midfields to protect that back 4.
Personally I don't rate Tevez, Ade, Bellamy, Elano or SWP are title side winning first teamers, but all are good enough for rotation/backup and all are a ridiculously large step up from the likes of Vassell/Hamann.