It's never too late for a good innovation and as old controllers get older retro gamers will be looking for a fix.

I don't know the nature of your joystick fix, but if it's something that can be done with off the shelf parts you will have a hard time selling it. Retro gamers are pretty good at modifying and maintaining their stuff and they take pride in doing it them self.

If your idea is a manufactured product you may find a small niche. But if it's something that can be easily copied it will spread across the internet like instructional videos on Retr0Bright.