When I started to play bass, I played fingerstyle only.
I had taken guitar lessons from a flamenco teacher and he taught me to use all fingers on my right hand (except for the pinky. don't know why). Hours and hours of training, alternating between the fingers (PAMI, PIAM, PMIA, etc).
It came in handy when I started to play bass in band that did Iron Maiden covers et cetera. Steve Harris was my idol back then ;-)
It wasn't after ten years in a band that I learned to play with a pick. I had started a tribute band doing Thin Lizzy songs. And Phil Lynott had this very steady pulsating right hand pick technique. Again hours and hours of practice to get that down pat.
I'm glad I learned both, because it just extends the palette of choice.
Never been much of a slapper though