Some of the best players in the Premier League have had some form of connection or other with Arsenal—Nicolas Anelka, Ashley Cole, Kole Toure, Yaya Toure, William Gallas, Emmanuel Adebayor, Samir Nasri, Gaël Clichy, Patrick Vieira, etc.
Others like Alexander Hleb, Marc Overmars, Mathieu Flamini, José Antonio Reyes, Thierry Henry and Cesc Febragas moved abroad to other clubs informed by one reason or the other—homesickness, ambition or greed, which—greed, that is—when tempered by the fundamental end of professionalism shades its most negative connotation.
There's sufficient argument to support the claim that a better yield in trophies would have been possible at Arsenal had a number of these players been retained longer than they were at Arsenal.
A huge factor, beside sheer inability to keep the player, is the profit that could be made in the sales of a number of these players, a factor necessary in the furnishing of the huge loan the club took to build its state-of-the-art stadium.
This came at a cost of a trophy or two, or even more.
In the following, I examine 10 of the players that could have made a difference—trophy-wise at Arsenal—had they stayed longer at the club.