Great topic! Here are a couple added nuances I think about frequently: Obviously the trails aren't the only environmental impact bikes have, the manufacturing of bikes and bike components of course also contribute. That Trek sustainability report from a year or two ago was really good at spurring that conversation.
Here is the best devil's advocate argument I can think up to justify mountain biking: Getting people access to nature, even if that nature is the backdrop to machine built jump lines, can inspire people to be advocates for conservation. If one lives their entire life in a man-made environment with little to no access to wilderness or nature how can we expect them to have enough of an emotional connection to that nature to want to preserve it?
Maybe, just maybe, if mountain bike culture really emphasizes sustainability within itself (buying used bikes, not upgrading every 2 seasons, repair not replace, sustainable trail building practices, biking to the trailhead, etc.) then use mtb organizations to educate bikers about environmental issues, there can be a net good. That's why it's so important to prioritize affordable, durable bikes and components and growing access to under-represented demographics in the sport, imo.