This works in most cases but "Will the work bring in more money than that person will cost" sometimes isn't so cut and dry.
Customer service is a great example of this. A good CS person is going to create value for you in the long run (retaining customers, and preserving relationships). Your still going to be aligning income to staffing, but the numbers may be a bit less tangible in some cases.
That's right but doesn't take into account lead time. Finding a good employee, especially early in the life of the company can take months. There is therefore a significant risk element involved in looking and planning ahead.
If the answer to all those is yes, it's time to hire. Otherwise, it's not.