I'm an avid mountain biker in this area. Here are some of my thoughts: 

Mountain bikers donate a LOT of time, money, and energy to maintain trails used by mountain bikers, equestrians, and hikers. Lawler Trail is a current example, as is the trail system being designed for Thurston. We're the ones going in with brushers and chainsaws to clear winter debris and trees. The MRT is an epic ride and not one we're going to quietly give up, nor should we have to. 

There has been a discussion about creating a parallel trail for mountain bikers on at least parts of the MRT because the trail does get congested. This is something we would embrace. 

Mountain bikers need to do a better job staying polite, keeping a safe speed, and following the IMBA trail yield priorities (mtn bikers yield to both hikers and equestrians). Those that do not practice politeness and safety threaten our ability to continue using trails and harm our sport. 

Finally, trail dogs should be leashed. Often, they're not.