When making a decision whether to undertake a (potentially) expensive or risky activity; decision-makers should always consider the CostOfDoingNothing. In other words, not only should the question "what happens if I do this project" should be asked, but the question "what happens if I don't?"
Occasionally, one encounters simplistic decision-making processes that assume the CostOfDoingNothing is zero; and in many cases it is. However, if it's a choice between enhancing an existing product and losing sales to a competitor; that latter option has a cost. It may be hard to quantify, but it's there.
It should be pointed out, though, that the "cost" of doing nothing is sometimes negative; in other words doing nothing (or doing something else--see OpportunityCost) has its own rewards. This too should be factored into any decision.
See SlackPatterns