I'm well aware of the Bush policy. It didn't go anywhere while he was president: how much influence do you think that decision has now that he's been out of office over two years?
There are two ways to implement the solution you mention, and both have serious problems. You can either build new trails and open them to only one user group, or you can close existing trails to current users. Building more crap in national parks will always meet at least skepticism, as it should in my opinion. And national parks attract a very different kind of visitor than do state or local parks. They are the "big deal" destination vacations, not where people just spend an afternoon. Are you willing to tell the couple from New York that even though they just drove over a thousand miles and planned their trip for a year, they can't hike the trail they've always wanted because they happen to be there on a Tuesday?
As to your final point, you will meet with two arguments, the first (in my opinion) more valid than they second. Firstly, mountain bikers are not a class of citizen, we are people who engage in the voluntary recreational activity of mountain biking. We as people are not excluded from our national parks, mountain biking itself is. So playing the victim isn't going to get you jack. And the activities you mentioned aren't allowed everywhere or even most places in many National Parks. Secondly, you will encounter the argument that mountain biking opens the gate for ORVs, and that association is the last thing this sport needs. The problem is that your argument does nothing to head this line of reasoning off. It is, after all, just as much the ATVer's park as it is the hiker or mountain biker, right?