For historical record purposes...
My first pair of 2X-S was in 2004, I sent that way back in a few months due to the cord oxidizing, and wanted to investigate to see if that's just something that happens with every 2X-S, or just the one I had. Turns out the clear wire just turns into a shade of green over time regardless, so that wasn't an issue. It wasn't broken or anything, it was just to see if this is something that happened normally with the clear cables.
My second pair lasted from 2004 to 2009, roughly full 5 years of usage out of it. This pair of 2X-S had the integrated wire w/ the "knot" inside the silicone casing, and I think what eventually happened is I tugged on the wire a bit too much, for some reason more on the right side. I can see a visible "space" I've created over the years of tugging, and there was a wire that was clearly broken leading from the knot into the circuit boards. So that pair finally broke after 5 years.
My third pair, with the field replaceable cables (with proprietary plug + nylon screw) was from the end of 2009 to now, 2012, so that was 3 years. The right-side driver died, to what I suspect is mostly moisture damage of some sort. This is probably also due to the fact that I don't have ear "wax" as much as ear "oil" and some bits seep in over time. This pair's construction had the deepest (also the straightest) tube out of all of the 2X-S I owned. So I'm guessing some oil actually made it through, of course I didn't realize that Sensaphonics had made new accessories to help deal with moisture issues, either with the dry & store system, or just the moisture wicks. I'll be sure to grab some of the accessories this time around.
So, really not counting the first pair, which was sent back in for investigative reasons only, I've had 2 pairs of Sensaphonics that lasted a total of 8 years. I think the silicone casing is more delicate than hard acrylic, because the casing is squeezable, and if you're rough with it when you put it into your ear and pull them out, there is a possibility that you could damaging them. On the other hand, soft silicone is very good at surviving against accidental drops, because they're soft and they'll absorb shock, they will almost never, ever break from a normal drop.
In terms of comfort, for me there's still nothing like soft silicone. With the hard acrylic IEMs, I always get some amount of seal breaking/separation if I moved my mouth while using them. No opening the mouth wide, no eating, and even laughing out loud will cause a temporary seal break. I think for some people, they probably don't think it's a big deal because they get used to it, and even hard acrylic molds are much more comfortable than universal IEMs. I have no idea how singers can deal with hard acrylic IEMs though, it's got to be annoying whenever they have to sing out loud. The opposite of that, for people who don't tend to sing with their music, hard acrylic IEM's fit is less "tight" in most cases, easier to put in and pull out, and maybe some people just don't like very snug feeling of the 2X-S.
As for mating hard acrylic w/ silicone to create some sort of a hybrid. I think the early UE's, and maybe even some companies now, made the "soft acrylic + hard acrylic" shells. In these shells, the outside half is hard acrylic, and the inside half is made from a heat-sensitive acrylic material that softens as it gets warmer, therefore it will be slightly softer and be just a little bit more comfortable. The problem was, the two halves can come apart at times because of the difference in the material's hardness. (On a side note, the UE5c I still have today, the soft acrylic part is a sticky, dirty mess... kinda nasty)
I would imagine if one were to make a acrylic + silicone hybrid, it will take quite a bit of work to make sure the two halves doesn't just separate over time due one half having flexibility, and the other half... not so much.
I got my impression done today, and my audiologist will be sending them in for me, I decided to stick to what I know; I love the 2X-S and will just be getting another one. I got to play with a 2X-S with the new Shure cable. The joint seems to be very stable, and the attachment point of the cable is much better than the old proprietary, field replaceable cable. If it wasn't for the ability to use Shure's iPhone cable, I would've still been tempted to get the integrated cable instead. The profile of of the ear piece is significantly reduced by the integrated cable, they fit almost completely flush with your ear with the old design. The new Shure cable is a very good compromise between aesthetics and usability.