Crazy thread, full of suppositions and assumptions. Hard to know where to begin.
1. BA drivers are, in fact, extremely reliable.
2. BA drivers ARE affixed to the shell in hollow-shell acrylics.
3. Acrylic shells are not especially fragile, though this is obviously something that varies by manufacturer and design. That said, they are more susceptible to breakage than silicone, which acts as a shock absorber.
Failure modes of custom earphones:
1. 80-90% of custom earphone failures are cable-related (based on years of product returns at Sensaphonics, which does not make acrylics).
2. BA driver failure is far more likely to be due to wiring issues than death of the driver itself.
The important thing to remember in caring for custom IEMs is never to pull them out by the cable. That is the fastest way to failure. Removal should be accomplished by grasping the shell to pull the earpiece out. Other than that (and regular cleaning of earwax), no special care is required.
Some notes on touring sound:
Pro touring musicians will not use products that don't meet their needs; the stakes are too high. The most important of these needs is reliability. Yes, they love free gear, but they won't use it on stage if they can't rely on it surviving the performance. IEMs are very different from instruments, amps, etc. Some IEM companies give a lot of product away for the marketing advantage that accrues (not naming names; should be obvious). This is more than a bit ironic, as it's usually the monitor engineer who makes the call on IEMs and, obviously, the big-name bands can afford anything they want. But there's a certain status in being endorsed to the hilt, and a lot of bands go that way. Similarly, the artist's management company may get involved, usually trying to save a few thousands by getting an endorsement. As a result, many of these artists are singing the praises of a product while never having experienced an alternative. So when they are told this is the best IEM ever, they tend to accept it as gospel unless/until there's a problem.
In my experience, there are a lot of people on Head-fi with more hands-on experience across IEM brands, even in customs. But the differences in application (on stage vs iPod) are significant, and should not be ignored.
FWIW, Sensaphonics doesn't play they endorsement game. Our endorsers speak on our behalf voluntarily, because they love the product and service we provide. They all pay, and they all pay the same price you would.
So my answer to the OP question is: No, custom IEMs are not especially fragile -- provided they were designed for the pro touring market (Sensaphonics, Ultimate, JH, Westone). I can't really extend that argument to other brands, most of which are designed for the consumer/audiophile market.