We all know that the door to door work is unbelievably inefficient and largely ineffective at getting new recruits in developed countries. We also know the reason for turning in slips each month is for the elders to assess which individuals are "spiritual" (in other words fully with the cult) and which are "weak".
I think the overall reason for continuing field service as we know it is to: 1) make a claim of being unique (not exactly true but whatever), 2) to reinforce an us vs them sort of attitude since most folks hate JWs coming to their doors, and 3) provide an easy method to assess whether or not men are committed enough to give them local leadership positions.
You could certainly figure out another way to accomplish #3, but #1 & 2 are much harder. You'd have to replace field service with an unequally uncomfortable task and I can't think of anything more uncomfortable than going to a strangers hous and peddling religion. And that's way the door to door work will not go away. Change? Yes, it already is changing. Tract campaigns, using the WTS website, etc. Go away? Not in my lifetime.