Charlie - sorry to be so definite on this, but this isn't a holdover from days of old. Yes, we can design in all kinds of protections etc. They all add cost, weight, power drain and complexity and they all lower reliability.
The real challenge is that we're talking about a 5watt burst on the 406 frequency - that's sufficient juice to fry things.
Some current ELTs will survive an open circuit for a period - I can't say with any certainty know how long each model might last. I know with certainty, based on field failures confirmed by bench tests, the Kannad Compact 406AF (not the Integra) is susceptible in this respect. The Integra is not, thanks to its integral antenna. Still, the field reliability of the Kannad products has been excellent when compared to other brands - this is why I still recommend the Kannad so highly when fellow aircraft builders come looking for advice. The three Kannad units under my direct care and control have been flawless.