It wouldn't be easy, I don't believe, and I'm generally quite negative about my own development abilities and values.

One point I couldn't fit into my original post is that I do have what I believe to be a good relationship with my business partner, but I feel he's disconnected from what valuable and capable developers make outside of his business. Also, he may well not be able to sell the business at all, and in that case he's suggested I could take over as manager (for higher pay), however I would be living with the potential sale hanging over my head, and not knowing whether this better-paid role would ever eventuate.

> this sounds like low maintenance software that the business owner can continue to sell for quite a long while without any ongoing engineering.

That's probably a fair statement, although there are normally updates we put out a few times a year due to changing business parameters in our market, and once every year we do a major release with new features.