I think a lot of it is environmentally-related. My first thought was that they maybe should put up some permanent cover along the way, and then I realized that it could be disastrous in the first hurricane or high winds. Bringing the pier closer to the island and dredging may have been a "no-go" from the Bahamian government due to potential damage to the ecosystem. The width may be of a similar issue - both environmental and issues with high winds/hurricanes. The bigger/wider the obstacle (pier), the more likely it is to be damaged due to a larger contact area. It could also be that this width had to conform to where it was feasible to put structural posts.
Benches may be a problem in high winds or waves. We saw some videos recently of high waves at Grand Cayman and at Coco Cay washing over the pier (washing over is a very mild description of what that looked like - it was shortly after our stop at GC was cancelled due to high winds), which would not only wash away any bench, even if made of concrete, but could potentially pick up those tied into the pier in some way and cause damage. Still, time will tell how they will deal with some of the drawbacks, complaints, and issues.
Castaway Cay looks very different today than it did when I first visited in 2000.
I really like the raised walkways. You're less likely to get lots of sand kicked up on the boards than if it was at ground level and it does not disrupt any "wildlife" (do you call crabs wildlife?) because they can burrow or crawl beneath without impact.