Well starting of with your question to how to focus on one field or idea. I would say just by thinking this kind of product would give xyz$ a year of something is really a bad practice.

Starting or working on an inception or idea requires huge amounts of market research and ability to analyse the effort you can contribute to make your product or service an everblooming money making machine. Even if you think of hardware and software product with AI and robots and while you might have great idea but if you don't give your efforts back to the business you won't end up well. Your efforts will come to great value when you decide and make decisions into what drives your interest. Even a mediocre idea can be turned into multi million dollar business.

The whole point lies in whether you would contribute vigorously to make your idea into money making machine. This will mainly depend upon your situation and decision taking capabilities. Start thinking on your situation to decide which idea will favour you the most and whether your interest lies into that idea.

And even if everything goes wrong with your decision, start working on another idea. You never know where that idea will end up taking you with.