20-Year Prospect
Let's be frank here—the longevity of your grant writing career is a bit more up to luck than it is to you. Sure, your writing skills play a part, especially when you write a winning grant...but for the most part, it comes down to chance.
If you don't end up writing too many winning proposals, you probably won't be employed for too long. Don't take it personally, most foundations fund less than half of the grant proposals they receive (source).
But don't fret. With so much writing experience, you're certain to be able to find a decent (and potentially less nerve-wracking) consulting gig. If not, there's always time to start the next Great American Novel.
Not to mention the fact that this job is nowhere near going extinct. For as long as society continues, people will still be pursuing the arts, research, and technology. And they'll probably want money for all of that.
Not to mention that if you secure some moolah for a non-profit or artist, they'll probably want to hold on to you pretty desperately. They might even refer you to friends and associates. Consider yourself their lucky rabbit foot.