I've been thinking about this post for the past few days, and you're really right. I never thought of the bass as a complex instrument, but I realized one thing....
It doesn't take forever to get your playing to a professional level. We look at guys and say I can't do that, that dude has been playing for 45years!...but the truth is, it's not how long you've been playing but how much you practice. I've seen people pick up an instrument, piano, bass, drums and within one year, you'd think they've been playing for 10! The problem that most of us have, me included, is that we don't practice enough, or we practice poorly--and this just get's harder with age. It's true that if you're young, you have all the time in the world to practice, but once you get older and get a family, you have to fit your practices in. In my opinion, someone that has the time to REALLY practice, meaning 6-8 hours a day, you can advance to a professional level of playing within a year or two...four at the most. It doesn't matter what instrument you play. Most of it is just teaching your brain and conditioning your body to the movements. Learning to improvise is a small part of it, because when you improvise, you're using bits of patterns that you already know, just pieced together to form something that seems random to an outsider. It's all about learning the patterns.