I wouldn't say, "killing" but battlefield effects are greater by air and arty at the operational level.
Air and arty funnel and attrit but do not defeat. (I say this as an Air Force weenie). You are however correct in stating that any force (light, medium, or heavy) is never so "deep" as to be out of reach of support.
I think the conventional roles of light, medium and heavy have changes. In WW2 the infantry was designed to penetrate the MDL and then the "heavy" armor would race through and move into deep or medium breakthru operations.
Now, I believe that heavy units are the one designated to open the hole in a conventional MDL. At that point is when a medium forces comes into its own. It now becomes the follow on exploitation unit. At that point light infantry can move through to widen the gap and provide flank security for the medium force with the heavy force now held in reserve.
I think everyone has a picture of a Stryker bridge hanging out 50km behind the lines on their own...when in reality they would be the tip of a larger force behind them.