You could definitely be right. But you're also looking at an extremely small sample size. 4 in the history of the UFC:
- Corey Hill - Was a .500 career fighter who was 2-2 at the time of his leg break. Hardly a good example. 4-7 after the break.
- Anderson - Was 38 at the time of his leg break. Hard to say whether age had just started to catch up with him.
- Weidman - Was 2-5 since (including) the title loss vs Rockhold (not including the Hall fight). Wasn't exactly setting the world on fire. Was in a steep decline prior to the break.
- Conor - 1-2 since going to box Floyd (not including Poirier 3), and looked to be on the way to a KO loss before the break.
The only one that numerically shows a serious decline is Anderson, but that arguably could just be him passing out of his prime. We would need a lot more up-and-coming and elite fighters at the early prime to break their legs to really make a solid impression of this.