Myself and my buddies do carry a little more than this, not much though. I see no reason to carry more kit.
If your multitool includes a chain breaker then you are probably right.
Well maintained bikes don't fail mid-ride.
Usually true.
I have had a few serious bike problems over the years but some of those were my own fault... A broken square taper crank was possibly caused by me overtightening a bolt. A broken seat clamp definitely was, and so was a stem bolt pulling the thread out of the stem body.
I truly don't understand how or when people have serious mechanicals that need fixing at the roadside. I never see it happen to me or anyone I ride with.
I have had spokes snap, and so have riders with me. It is usually possibly to get the bike rideable just using a multitool.
I have had and seen multiple broken chains. Chain tool time!
Lost bolts, jockey wheels... Whoever didn't tighten them properly!
Fork failure due to rust... That didn't happen overnight and should have been spotted developing by the bike's owner.
Tyre worn through by a brake block rubbing against the tyre wall - adjust brakes properly.
Seatpost snapping! That one was a freak failure that happened on a mate's bike. He nearly gave himself rectal surgery on the broken end. There wasn't enough post left to get the saddle to a decent height so he rode back to the car standing up, with the saddle in his jersey pocket.
Rear mech into wheel causing catastrophic damage. We managed to remove the mech, shorten and rejoin the chain to create a singlespeed bike, straighten the wheel enough to ride it, and take the rear brake off because the wheel wasn't straight enough for
that! All of those jobs done with a multitool. (The problem was caused by airport baggage handlers dropping the bike and bending the mech hanger. The owner had straightened it by hand at the hotel but the hanger failed out on the road.)