You may be underestimating yourself or overestimating the degree to which startups need rockstar engineering. If you ship and it works, you're good enough. Better is better, but most engineers can create plenty of value without being e.g. DHH or Linus.

I was in similar straits at the old job: I was one of the more trusted engineers, spending my time doing maintenance coding on an evolutionary dead end. I used it as an opportunity to bone up on my general web dev skills (System architecture, design, SQL, etc). I also used my "routinely delivers on promises" karma to buy changes to the development roadmap that were useful to my professional growth. ("This project would be improved by integrating Prototype. I have taken the liberty of mocking up a screen. We should do more like this, the clients will be thrilled. Of course, you'll need to assign me to it since I am one of the only two competent JS engineers here and the other is busy.")