A different position would be to simply let these misconfigured and badly written networking programs fail - and let them fail hard.

Yes, it will be painful in the short term and users will notice outages, but in the long run it will result in these issues being fixed. If we continue a policy of reversion to bad workarounds (which is really what JSONP is), we not only add complexity and the potential for obscure exploits to our own software, we ensure that nobody at Cisco et al will ever feel the pressure to fix these bugs.