There is not much to add after what theymos said, but I just want to point out that while our attention is often captured by those high-profile cases - Legendary, DT members, etc - the vast majority of banned users are newbies and other low ranks, and that has always been the case.

Or to put another way - the fact that those older users managed to get away with it years ago should be viewed as an accident, not a free pass, and the sig ban option is already a huge privilege for them.