I'm in a band that l used to play guitar in but am now playing bass. I'm naturally good with the instrument and  understand the nuances of playing decently. I'm not amazing but I'm good enough to play what most people consider hard bass songs.

This guy goes on vacation to a tropical island. As soon as he gets off the plane, he hears drums. He thinks "Wow, this is cool." He goes to the beach, he hears the drums, he eats lunch, he hears drums, he goes to a luau, he hears drums. He tries to go to sleep, yet he hears drums.

This goes on for several nights, and gets to the point where the guy can't sleep at night because of the drums. Finally, he goes down to the front desk.

When he gets there, he asks the manager, "Hey! What's with these drums. Don't they ever stop? I can't get any sleep."

The manager says, "No! Drums must never stop. It's very bad if drums stop."

"Why?"

"When drums stop...bass solo begins."

The problem is, with notable exceptions, the bass just isnt a lead instrument.

Agreed. I was a professional bassist for, well, almost 2 decades. Your job is to be a rock-solid part of the backbone, not half-guitar it.
That's why there are so few actual bassists vs. guitarists playing bass.

As others have pointed out bass isn't really an instrument to play sustained solos on, and outside of the jazz world the majority of the bass solos I hear tend to be lame and embarrassing...

That said, what helps me with soloing is to keep the melody of whatever song I am playing in my head...

ill just leave this here...one of my favorite bass solos...skip to 5:20

sorry, dont know how to embed..

solo

 

 

So am I. No "art of good bass playing" has been lost, but there are a ton pf people who are guitarists who have no idea where the instrument fits in a band.
Jam bands are horrible abominations of sounds and are not music, and almost every band you mentioned with the exception of Cream is a good example of how not to play.

Unless you totally throw everything out & go Les Claypool, you don't play a four string rhythm section instrument like a 6 string lead.

Good bassist: http://en.wikipedia.org/wiki/Carol_Kaye

Bad bassist:

So now I am confused...Are you asking us how to actually solo with minimal or no support of the band like a jazz player would do?  Or are you asking us how to accompany a guitar player while he solos??