I don't know what proportion of the illegal dirt bike riding is on plow lines, compared to how much is on newly-created dirt bike trails, but, regardless, illegal is illegal. Eliminate, no, but there are ways to get it better under control. And yes, I do blame the bikers. They are violating state regs. And the damage they do has not been evaluated, but the noise, visual stimuli, air pollution, and other impacts are undoubtedly a big problem for wildlife. Is it a lesser amount, compared to the ATV traffic and the illegal 4WD traffic (i.e., people leaving the road system to drive in wildlife habitat)? That may be true.
I don't see how you ever will get it under control. It is what it is and will continue to be that way.
Everything you and others are concerned with is wildlife and plants. I am not saying that I don't agree, I am just saying in order to protect them as you want will require now legal roads to be closed. I am against that and always will be. As you can tell I am a one issue voter.