A vibrating string on your bass has two ends. One is anchored in a relatively large clock of metal, which is screwed to a large block of wood. The other is held by a finger against a tiny strip of metal, which is anchored in a relatively thin strip of wood. If you want sustain, you want the energy to remain in the string, not leave it. Energy leaves the string by 3 methods:
1) Vibrating the air around it.
2) Vibrating the wood in the body
3) Vibrating the wood in the neck
If you want increased sustain, you can't do anything about #1, other than playing in a vacuum, which has negative health effects for the player. For now, let's move on.
So we're down to $2 and #3. As a Physicist and an Engineer, the thing you do to improve something is look for the place where the most compromise exists in the design and improve that. In the case of en electric bass guitar, that place is not the bridge. More sustain is available, but to get it, you fix the other end - anyone who has owned a Kramer metal neck bass (I've had 3) can tell you that is has undeniably more sustain than a wood necked bass. It's not a minor change where people argue about whether or not its audible - it's OBVIOUS. Why? Because they fixed the part of the design where the most compromise exists - the neck. Yes, they neck dive, but if you're young and strong enough to deal with holding up a neck all night ( I was in my relative youth), it's a great instrument.