It is not easy to kill well established products. We are ages away from UBB. UBB was not similar to vBulletin's current position as a brand name. Surely for active webmasters , they can realize differences between old and new vBulletin versions but many people are not exprienced and they will not easily see the differences between old vB and new vB. vBulletin will always remain within top 3 bbs systems out there even under IB management because of the Big Board number that uses vBulletin.
more importantly much lighter. xf loads very quickly! its one of the fastest scripts that i dealt with, but then again, speed is a combination of server power + conf + the script itself