"Wow ! I can't imagine how thin and delicate that reed must be in order for that little magnet to close or open it. Are they suseptable to damage over time because of what i would assume is a very delicate construction? I'm thinking it must be like tin foil as far as lightness and strength go."

The reeds are encapsulated in glass, specially gold or iridium plated contacts, and are very, very fast, (I've observed them operating in microseconds) sensitive and rugged. Of course there are many different types and I obviously don't know them all. Good ones will outlive all our bikes.
These were used in phone switching equipment with millions of relays and millions of operations - extremely dependable. They were/are also used in airspace equipment as sensors, in redundant mode of course. My experience with them is that they may more likely develop residual magnetism and "stick", rather than not operating - in a good design. On a "shaking" bicycle there should be zero problem with "sticking" so they are extremely reliable in this application. My only problems involve either getting the distance between it and the magnet too large (fix both the magnet and the sensor really well) or weak battery, effecting the "transmitting" range. With wireless system you'll not have the fragile wire hassle, but it requires battery is havier and does have minor interference and range issues. Mine are rarely unreliable near power transformers and traffic signal controllers, if I stop in their proximity.