On a basic level, the 34 is actually must more creative than the traditional 43 because the linebackers can do anything.
But generally we need to get away from discussing 34 vs. 43. It’s about the individual players in the front 6 or 7 and what their responsibilities are. Belichick drove this evolution, where in the past you had a 34 with the front 3 two-gapping and the 4 LBs making the plays.
That’s just too simple of a defensive schematic now.
Instead you’ll have variations across the front of who is two-gapping and who is single-gapping and it will change from down to down. If you drew a line down the middle of the defense you might see one side look like a 34 while the other side is a 43.
The way I breakdown the Patriots defense now is you have your two edge players who are pure DE/OLB hybrids who must contain but will drop into coverage occasionally (less than tradition 34 OLBs).
Then you have your two defensive tackles, where you want a diverse skillset so that you can run two two-gappers (Wilfork/Siliga), a two gapper and an upfield one gapper (Wilfork/Chris Jones (or hopefully Easley in 2015), or two upfield guys for pass rush situations (Chris Jones/Easley).
Then you have your linebackers who, like the DTs, you want to be diverse. But in today’s NFL, they have to be able to run, all of them. That’s why Brandon Spikes couldn’t be a long term fit, despite being an elite run stopper.
What it boils down to is having enough versatility in your front seven players to mix-and-match and play whatever game plan you need to stop any particular offense.
The 34 will always be the defense Bill Belichick teaches out of in training camp and occasionally they might go to it to stop run-heavy teams. It’s what he teaches the calls, checks and gap responsibilities out of, but that regular 34 defense just doesn’t give modern offenses enough problems. It’s easier to block and it doesn’t generate enough pass rush (because the front 3 are all two-gapping and rarely causing disruption and getting pressure) to be effective in a pass-heavy league.