It won't die. It's after all not a radio-focused extention (despite the use by radio stations), but the ccTLD of an actual country. As long as that country exists, .fm will exist. At most it will become a rarely used extention, but it will remain in existance.
I am a member on a similar forum as this, but for discussing email providers. One of the most praised providers on that forum is FastMail. They have many extentions to choose from, but their main domain uses .fm as extention. This is not a small service, they have many users, of whom quite a lot choose the main domain including the .fm extention.
Also, if the domain would drop drastically in pricing, then I would also see a rise in new registrations. Radio stations still need a domain, and many new radio stations are being founded in recent years in my country (I assume this applies to many more countries), on top of that the extention would be great for podcasts too. However, the costs of buying an .fm domain need to go down before it will be more widely used.
I think .tv is a bit in the same category, the possibilities are huge (not just for TV stations, but for any video- and photo-related website) but the price tag scares off some people.