Your approach to mtn biking sounds like my approach to 4-wheeling. I have spare fluids, spare parts, and tools, honed over the last 10 years of four wheeling and wrenching on my rig. People chuckle at me as the guy who is over the top prepared, but it is all in good fun because we all know I'm the guy they borrow stuff from when their vehicle breaks. Not that I carry everything or think of everything. I've had to borrow this or that from others. And I learn what to add to my stuff for next time

I'm usually reasonbly prepared to get stuck overnight on the trail.
Thinking about this just now -- it hit me.
I prepare for the most expected SHTF outcomes of my recreation and activities.
Not for some outlandish scenario that has a 0.0001 probability of happening given the activity at hand (ie, having to stay 2 weeks in the wilderness while fly fishing a stream 100 yards from the highway)
When I four wheel I typically have enough stuff to make it through a night or two. I always carry the big first aid kit in the Jeep when I go camping, wheeling, or fishing. I never go alone, so there's always contingencies if bad stuff happens. I always wear clothes that have pockets for the basics. Could I prepare better? Sure.
When I go hunting I carry basic stuff, but am never all that far from car or civilization (at least not yet). I think the worst that could happen is getting caught in a blizzard, but the truck is always within a pretty short distance. Could I do better to prepare? Maybe so.
When I go camping I bring all the stuff to stay warm, hydrated, fed, happy. So if a freak blizzard rolls in or massive rainstorm, or am otherwise stuck 'out there' (altho I never seem to camp alone...) I should be good for days. Could I be better prepared? Could be.
The point is... as one gets farther away from help, one prepares for more extreme scenarios. This is a more 'organic' approach. Your preparation is based more on experience and can be more focused.
For example, I tend to carry more spare parts (the ones I know are more likely to crap out in my rig or others') when going to Moab or Ouray because I want to maximize my chances of getting home in the vehicle I drove out there. If I ever start backpacking, or mountaineering, or hunting in remote locations, it would be a different ballgame of preparation. I'd put together a kit that would let me ride out bad weather and/or being lost for several days or more.
The preparation ends up being an outgrowth of activity/hobby/experience. So that in mind I need to make sure I have the right kits for different activities in a form that is easy to take with. Best to get out of the mindset of "it's just a _____" (short hike, quick four wheeling trip, etc) and get into the mindset of "what if I have to stay overnight or even past dark". I've been stuck on the trail in the mountains hiking after dark fairly unprepared a long time ago. Not fun. What you say about the cascading of events is brilliant.
Thanks for the ideas for what you bring and how you carry it, and thanks to all so far for helping me "talk" through this.