The manner in which the War Index is calculated has changed. C3s used to have only 60 WI points . . . after painting the map and pummeling the Capital and Cities, there didn't used to be much standing in the way, a few points here and there. You'd just need to hit a few corps and towns to win.
Nowadays, there are 100 points. Forts used to be worth a solid .5 points and add .5 points to the WI. Now all of them combined are worth 50 points with each one taking up a fraction of the points (say you have 5 Forts, each is worth 10 points). Corps are now calculated differently, first all of them are now worth 20 points instead of the old 10. Second, it used to be that for a 10M pop C3, 10 corps each made up 1 point of the WI. If the C3 had 9 corps, it lost a point at the war's start. If it had 11 corps, the first corps' destruction didn't effect the WI as much (only through pop losses, not through the corp loss).
All of this put together means that there are 40 more WI points to get rid of. You need to destroy more of anything to win. Forts will drop the WI more than before. Corps will drop the WI more than before. However, there is simply 40 more points that make the war take more destruction.
Nowadays, I'd say that you need to hit the Capital, Corporations, Cities, Towns, Paint, and carve off about 1/3 of the Forts to win.