I've seen you mention "reverse engineering .net" about three times now.

All I have to say is "web services." If you're not going to obfuscate it, and you're on a LAN anyway; set up a service that houses all of that critical functionality and have users subscribe.

---IMO---
If you don't want to upgrade, then be honest with your boss and say just that. If he/she asks why, just say that it's a training issue or flat out tell him/her it's just not in your best interests. If you haven't used the Fx first-hand, don't rely on a bunch of strategically picked articles to develop an opinion.

If .Net was as insecure and unsafe as your facts sheet make it sound, how come we haven't heard of any company getting nailed with an exploit that set them back millions of dollars? Or even identity theft (not directly relating to hardware theft or user misconfigurations).

Since the the Fx came out half a decade ago; the only negative articles are a bunch of pestimistic, nostradamus-like predictions. A wise man once said, "Don't dog it till you try it."