The above comment is really valuable, for sure. But the way you said it let me think you are more afraid about the risk of being copied by someone bigger than you. And due to the beginning (the "So let's say..."), I wonder if you are a startup founder evaluating current funding opportunity, or a future founder thinking about different ways...
I want to focus on the risk of being copied. First, in a general manner, you can't know if another human being would like to copy what you do. Probably yes. The interesting questions are more: can they do it ? and is it financially rewarding for them ?
First question: can they do it: often, it is only a question of money. If you have really deep pockets, you can copy any startup and kill it. But is it economically reasonable ? That is the question. It is directly in relationship with your activity. If you do something highly technological, long, hard and expensive to replicate, and if the currently visible market is relatively small, nobody will throw large amounts of money to this problem, even if you can make profit on this market. You do because you are small and talented, but being rich won't help.
If you do something based on network effect (think Twitter or Facebook), you become untouchable when you have a critical mass. With exponential growth, each day passing without competitor makes you stronger, and increases the entry ticket. It diminishes the potential profits and increases the risks for a potential competitor.
If you have no technology and no network effect, and you are solely based on an innovative business model or a new way to market something, there are chances somebody will think he/she/they can do it better than you. So you will need exceptional operational execution at all times to discourage competitors. In this case, well... good luck !