Looking at martial arts like this, the real question seems to be why doesn't everyone quit?
First you have to get everyone to agree to look at martial arts as your hypothetical juggler does. I never have and know I would not look at all martial arts that way. But,
There's a huge range out there, in martial arts, as with most pursuits. You just have to find what suits you.
So some pursuits that call themselves martial arts would not qualify as martial arts imho, and I think I could identify them quickly enough I wouldn't need to quit since I wouldn't begin them.