Defenses have also evolved to do this better out of a 4-3. You look at the best modern day 3-4 pass rushers and there is zero question about who is rushing and who is dropping into coverage. Pittsburgh is going to rush TJ Watt 99 times out of 100. The Cowboys are not going to move to a 3-4 and start dropping Micah Parsons into coverage a third of the time the way the would do an Anthony Spencer back in the day. It wasnt even uncommon for DWare to get 100+ coverage snaps a season.The success of a 3-4 pass rush is predicated on the surprise element of that 4th pass rusher. He could come out of nowhere. The defense is looking to confuse the O-line and their blocking scheme.