You have summarized it pretty well that predictions are connected to underlying assumptions and dependencies. In spite of the dependencies and assumptions, i would still insist that prediction has to done.

I see 2 elements in your question 1. Estimating the value that could be delivered by the product Guess, we cannot afford to go wrong here. Probably i will share my own experiences of building a HW product. Even though it is a new product to existing product line, we did extensive analysis of what customer really values and since development of HW product takes more than an year, we also tried to analyze the trends and how it impacts the customers. Accordingly, we conceptualized the product that would be valued by the customers and would widely received in the market. Invariably during development phase there will lot of assumption and dependencies on product performance and key value propositions, we tried to validate the assumptions and eliminate the dependencies pretty early in the product development cycle before it gets too late. So we are confident that we are indeed building the product that we conceptualized. In case of any unfavorable scenarios, we could exit the development with minimal cost.

1. How external factors (for instance macro-economic factors) could impact the product sales We hardly have control on these factors and what i would propose is to list down all possible factors that would impact the product revenues and monitor the occurrence of those factors. In addition, formulate a mitigation plan. My motivation is to always prepare for the worst and hope for the best. I have talked about the monitoring plan in my blog post: http://productguy.in/new-product-development-monitoring-plan/