I'll admit up front that players, not scheme, will utlimately determine the success of a defense. But if you ask me, that's a rather shallow counterpoint against switching schemes. Obviously, if you line up a bunch of scrubs you aren't going to be successful. But to blow that into "scheme doesn't matter," seems silly.
There's a reason why the 3-4 defense exists. There's a reason why some coordinators live and die by it. There's a reason why some of the most dominant defenses in the last twenty years have employed it (even if they didn't need to do so to be dominant).
Here I'll list some basic advantages to a 3-4 scheme:
Variability: The 3-4 scheme creates confusion, leading to imbalances on the O-line. You'll often find one side overblocked with someone else unaccounted for. Delayed blitzes from the 3-4 are much more effective because the o-line often overdedicates itself to the 3 down lineman, particularly if you have a penetrating NT that requires a piece of everyones attention.
Additional ILB: Allows for much more diverse font-seven blitz package (outside blitzes and inside blitzes, single and double).
Additional Spread: More guys outside of the tackles puts you in position to defending outside running plays and screens.
Easy to Find "Tweeners" and "Space Eaters": 4-3 DE's have to be polished pass rushers with good technique or freakish athelticism. That's a rare find. Many rookie DE's are simply too small to have their hand on the ground and at that point in their career they are about as fast as they are going to get. We like to call them "tweeners" but they are really the rule, not the exception.
It's easy to find a stop-gap NT. You can get away with plugging a really big fat guy in there for first and second down.
Now suppose you believe that what I've said is true, but because the Bills are devoid of suitable front seven candidates for that defense NOW, they shouldn't make a switch. A point that I agree with, to a certain extent. However, I don't see this as an all-or-nothing proposition.
I do believe the Bills should begin to move towards a base 3-4, but because of their current personell I think it would be wise to employ a hyrbid for the 2010 season and slowly draft the defense towards a typical true 3-4.
It will take some time, and there will be growing pains of course. Not every piece will be available this offseason or next, but within a couple of years we won't be talking about this defense as "scrappy," we'll instead be talking about them as "dominant."
Do we HAVE TO switch? No. 4-3 defenses have succeeded in this league. Is it more likely to succeed, over the long haul, if we move in the direction of a 3-4? I belive the answer to that is yes, for the reasons I highlighted above.
From what we've heard from Gailey and Nix so far, it seems they agree with me.