Heres the thing. People are very particular about their headphones. People are not going to just by your replaceable earbud headphones because they can replace the earbud if and when it breaks.
That being said, your idea is by no means a bad one, but it would be better if you approached it a different way.
First and foremost, what your talking about had already been made possible, but there currently is no market for it. Any headphone that utilizes a detachable cable can technically have that "earbud" replaced as well (assuming a fit to the cable). However, right now this technology is generally available in very high end headphones, and the solution was designed to fix chord breakage problems.
Now assuming we are beyond our warranty's limitations, it would be a cheaper solution for the consumer to replace one earphone instead of the entire package.
However, that is precisely why the major brands don't sell you a single earphone replacement- because they want to sell you another entire set.
So what you need to do is convince a major brand to start offering single earbud replacements. It would have to be marketed very well and be a major factor in a person choosing that brand, or it would in all likelihood actually reduce revenues.
If I were you, I would appeal to the Eco Sustainability market (Green). Single bud replacements would result in less wasted material, as the headphones would be essentially fixed without the extra waste of an entire new set. This reduces carbon footprint and all that other good stuff hippies like.
Hope this helped or at least provided some outside insight
-Max
