IMO, there is a vast difference between the style of cycling justifiable on a closed course, such as a MTB park, and the type of cycling justifiable on a multi-use public trail.
Same as with use of a motorized vehicle. On public roads where it's likely to encounter other drivers, there's good reason there are rules and regs, and why horns and lights for notification and warning make sense.
I, too, have had reckless folks engaging in a variety of modes of travel (whether walking, walking with pets, jogging, skateboarding, cycling or small electric/motor bikes) who have recklessly disregarded the fact others were around.
I'm all for bells and horns on bikes, if people are regularly going to be on multi-use, multi-person public trails.
I'm adamant about riding/jogging/walking in a manner suitable for being around others. A solid percentage of folks out there, however, don't seem to do that. Wish that weren't the case. For many of them, I can't see a bell or horn helping much. Particularly in the case of folks whose hearing is overwhelmed by whatever earbud-related activity they've got going.
That said, it is what it is. We can do what we can to ride at sane speeds when there's a likelihood of others being around. So can others. Horns and bells, or not.