You are looking at things too black and white. You can be capable, taking care of your role on the field and result in performing well without being an Evra. Of course it'd be nice to get a Maicon type, but it's not a case of you are either world class or you are garbage and Beck and Zeigler would be solid acquisitions. If we had a money shitting Sheik then by all means, go insane with transfers, but we don't and we need to do something regarding the left wing and a proper heir to DP.
The market perception is very important. When you get players of the stature of Aguero you are sending a message that you really are planning to get back to winning ways and will target great players to do so. That makes us more attractive and more likely for talented players to join us, which we badly need considering how our reputation has been bruised these past two years. Talent attracts talent, winning attracts winners.