Word was that the UFC paid Lesnar more than anyone else in their history for a single fight. At the time it probably looked as if they were reinforcing an already great card but with this development I think UFC probably expected Jones to fall off and needed a huge fight that could slip into the main event.
I agree, it's such a stacked card that even losing the main event there are still loads of great fights with known names that make is an easy sell.
However, as you say, if anyone else falls out then suddenly the prestige of the card could fade very quickly.
you always have to expect that.. injuries in training before a fight is very common