It absolutely can be fatal. Life is fragile.

I mean sure I guess, but so can walking across the street if we're defining "fragility" and "risk" only by what's technically possible and not what's likely. That definition may be valid semantically, but it's useless in the context of discussing how to mitigate risk because it's so all-encompassing that it would include nearly every activity that involves leaving the house (and even many that don't). And if you insist on assessing risk like that then it seems like it would have to follow that mountain biking must ALREADY be an incredibly dangerous sport even when everyone is following all the rules. For the more "extreme" sports like this, the danger/challenge is literally a large component of the fun. And for mountain biking, a large component of that challenge is due to the exact sorts of "deadly obstacles" you decry like boulders, rocks, branches, roots, holes, etc. So there are mountain bikers out there for whom these obstacles are fun / thrilling to avoid when they are in their "expected" spots in nature, but if they ever end up in an unexpected location like across a trail, they apparently go from being a fun challenge to making the activity deadly and an unacceptable risk.

If all it takes to push your sport over the brink from "acceptably safe thrilling fun" to "unacceptable risk to my life" is something as routine and unpredictable as a rockslide, a washout, or a rotten tree falling across a trail, then I would seriously re-evaluate whether it was ever truly "acceptably safe". But again, I'm skeptical there are really people out there like this. I mean, I definitely know people whose sense of personal physical safety very strongly depends on things being in their expected locations, but these people would never even dream of riding a mountain bike on trails where their presence might be unexpected to others