He is tailor made for the position he plays in now. If you look at average positions of our players for pretty much every game, then you will see that Walcott is THE most forward player. Even more so than Giroud. For all intents and purposes, Walcott plays as a 'striker', rather than a 'midfielder'. IMO, he would get less opportunities to get in behind the back 4 if his starting position is that of an outright striker, thus negatingTHE outstanding quality of his game - his pace.
In attack and in offensive transitions, we currently play a lopsided 4-4-2 formation. Walcott becomes a striker, and the LW player becomes a midfielder. Once we have managed a defensive transition to our 'normal' defending, Walcott slots back into midfield and we play a 4-5-1 formation. Best way to describe it I think, although slightly simplified.