I would have agreed with you if only his career was going at the right direction with Bordeoux. Unfortunately its evident that it wasn't the case and that is reflected by what was said by Blanc (he need to strengthen his mentality bs) and the fact that he was loaned.
The EPL is a tougher league and players are allowed less time to develop then in France. Stating that it fair to say that it developed many French players in the past. Players like Henry, Viera, Silvestre, Anelka and Clichy all made a name in England. In certain cases (im referring to Henry, Viera and Mickey) it ended up saving their career.
As stated the EPL is a tougher league and Obertan will have less chance to develop than if he stayed in France. On the other hand he will be training with some of the best players of the world and coached by a manager with a CV which the French League can only dream of having.