The point of copyright is to offer a limited monopoly to encourage making risky things. If you can't or don't want to monetize it, then you don't need the monopoly, and it's unlikely that such a scheme would stifle innovation.

I can't imagine someone saying, "I want to work on this idea, but I won't because I know it won't be profitable for many years".

So start the clock after a (small) number of initial years. And/or make it free until publication (so e.g. making the rounds with publishers doesn't require you to start paying). Or tweak the exponents.