How many times were we actually in a base 3-4 defense with most teams in the spread before the coaching change?
Whether we were in a nickel 3-3-5 or 3-4, or some other exotic alignment...we failed to get enough size, speed and technique on the field, and it's been woefully apparent for awhile now! IMO changing to a 4-2-5 alone will not fix that issue...but it will buy Coach Schmidt some time in the court of public opinion based on fan reaction I see...
Alabama ran a 3-4 defense under Nick Saban...technically Georgia is too...but what a 3-4 was 20 years ago isn't the same as it is now.
Any scheme can be successful if you have the right players to run it.