an entrepreneur would only assume the rather significant risk if the likelihood of payout (as denoted by your clever equations)is high enough. in a capitalistic society such as ours with millions of established businesses that would entail either 1 of these two options.
1- Make the way things are done normally, better. (Fedex vs USPS)
2-Do a completely new different thing (Facebook.) Both options are disruptive, although at different levels.
And I think you're conflating competition with disruption.
Entrepreneurship and business go hand in hand, you can't have the one without the other. Someone that risks their personal fortune to open a hamburger stand by your definition is not an entrepreneur, by my definition he or she is. And that's before we look at the possible futures where that hamburger stand ends up as a chain of stores because he innovated on quality or was exceptional in service.
Disruption is not Fedex vs USPS, it's not Facebook competing with myspace for eyeballs. Disruption is AirBNB threatening the established interests in the hospitality industry, napster making record execs very worried and EasyJet causing increased blood pressure with all their fat cat competitors.
Disruption by definition can only come when a market is established, but it is definitely not a prerequisite for entrepreneurship or running a successful business.
There are endless possibilities out there to make a business and most of them are not disruptive at all. The disruptive ones are vastly more risky.
No, it really isn't. It's about getting $Y out for every $X in where Y > X, and doing that in a sustainable and preferably an ethical manner.
Disruption is the exception, not the rule in entrepreneurship, and success is simply (Y/X) * volume processed * lifespan of project. Some would even say that success is the net cumulative effect on society but that's a lot harder to measure. You could for instance make this about jobs created or amount donated to charity or something like that but most people simply use the formula above or some variation on it.