I have also heard the argument recently from somebody with an engineering background that resonance and sustain (two concepts that I had long assumed to be complementary to each other) might actually be opposing in some ways. The idea being that in order for a string to sustain for a maximum amount of time, you would actually want it to be separate from the resonance of a bass neck and body -- in the sense that the string is more perfectly/consistently constructed than the organic and inherently imperfect wood. So if no outside force is hindering or dampening the string, it will ring for a maximum amount of time. The natural resonant qualities of any piece of wood are imperfect, so once those resonances start pulsing, they will affect and interfere with the string's closer-to-perfect vibration. That's not to say that a high mass metal bridge won't improve sustain, maybe even the exact opposite. If you take an Alembic bridge for example, that brass bridge is anchored in a big brass block that is sunken into the body. That brass block could actually be serving to even-out/disperse/decrease the amount of wood resonance interfering with the vibration of the string, while simultaneously giving the string the most solid, but resonantly-neutral anchor point possible.
I do not have an engineering or physics background and this is me paraphrasing so I hope I'm doing this idea a disservice. The gist of the concept is just that vibration transfer from string to body is actually detrimental to sustain. That said, resonance transfer from string to body is not necessarily a bad thing as it can enhance certain frequencies, making the notes rounder, boomier, etc. (but simultaneously decreasing the amount of sustain). Think about the big booming, resonant, warm tone of an upright... but then think about the sustain of an upright. Also, consider that Alembic makes multi-laminate necks, specifically to make the neck stiffer/sturdier (going back to creating a solid anchor/host surface for the string), but also to interrupt the individual resonant qualities of each piece of wood, which minimizes the wolf-notes and dead-spots characteristic in a solid 1-piece maple neck design like we see on Fenders for example.
So high mass may increase sustain, but not because of contact to the body, simply because it's a very non-resonant barrier between the imperfect wood and the near-perfect string.