They have but there's plenty of clubs that have had a season or two in the sun before fading away again. The ones that actually turn their success into a continuous pattern of trophies are the exception to the rule, the majority get stuck in a cycle of: produce good players-> do well-> sell good players -> develop new players -> do well ->sell good players-> develop new players etc etc. All it takes, without the financial backing that clubs like United already have, is a bad season or two and it all comes crashing down.
Personally I would like to see stricter rules put in place to curb spending, but I wouldn't differentiate to such an extent. Surely the sheer amount of money being spent is as much of an issue as its origins.
Yes there are many clubs who follow that cycle but I was just pointing out the clubs that have improved and challenged the best without any external help which City and Chelsea fans (not all) used as an excuse that without sugar daddy clubs can't challenge big clubs when Atletico have challenged two of the biggest clubs in the world and Dortmund did the same in Germany.