Thanks for the question. I certainly recognize this conundrum.

My response would be that it all depends on whether these small incremental changes are actually making a significant and tangible improvement to your customers and to your business… And the only way to answer this question with any certainty is to have clear metrics defined up front that support your specific product, customer set, and business model.

The approach set out by the Lean startup movement would be to only put time and effort into activities that you can quickly and cheaply test. So in this sense, endless tinkering just because someone has a new idea is not advised (because you have no way of knowing whether the effort will be successful or wasteful).

However, if you have a clear, measurable goal — say, your aim is to reduce the time it takes for a customer to complete a particular work flow, then you might well try out possible changes with a subset of customers (e.g. through A/B testing or through user testing). The data you get back from these experiments then shows whether your proposed changes are actually achieving the stated goal or not (and can then be deployed more broadly or scrapped). Either way, you will have learned sometihing and can act with confidence.

Regarding the opportunity cost you refer to (whether to keep working on this product or whether to put your resources into a new product), again the safest way of making that call would be to set up some quick, cheap experiments that test the validity of your new business / product ideas.

I hope this helps.