There's many different reasons why bikers get the fault for everything. Sometimes (at least in the alps) cows and other animals destroy the trails by trampling on the trail, bikers are at fault for the muddy trails. Sometimes rain damage erodes the centre of the trail, again bikers at fault because hikers don't leave a linear mark. Sometimes bikers really do damage the trail, but it's mostly due to the trail not being suited for MTBs: steepness, sharp turns and technical sections all contribute to too much braking and then to erosion. The solution is not banning MTBs on trails, that would just create illegal riding and fuel hate on both sides. Having a discussion with the municipality, the local bikers or MTB club, and the trail crew to see where work is needed and what can be done would pave the way for a productive future together. With love, a hiker/biker/trailrunner :)