In the climbing world ethics and respect for the environment are ingrained in everyone, young or old. You don’t see young turk climbers chipping holds or bolting grit, or dropping litter or acting like dicks at the crag. And when someone does step out of line they are quickly shut down by the wider community. We probably need a bit/lot of that in mountain biking.
Whilst there are issues within climbing, like litter at the crag (finger tape etc, so clearly climbers), human crap, chalk and tick lines everywhere and even music, I think that the smaller overall numbers lessen the impact.
I think that the key point you make is “when someone does step out of line they are quickly shut down by the wider community”. In mountain biking I kind of get the feeling that the wider ‘community’ doesn’t give a crap about much except their latest youtube upload or strava segment. Of course, mountain biking has moved on immensely from the rambling on two wheels of old, and that is inevitable, but it is certainly putting a lot more pressure on the environment than was present 20 years ago.
Some people and groups, such as Ride Sheffield, do fantastic work, but mountain biking lacks any cohesive body to both represent and influence riders in UK.