Quite easy. Bitcoin has won if a "significant" number of people (say, hundreds of millions) use it as a daily currency.

Meaning:
. Salaries partly or fully paid in BTC
. Taxes paid (or at least allowed to be paid) in BTC
. Most expenses/shopping paid in BTC (food, transport, utilities)

As long as Bitcoin is seen mostly as an "investment", a trading asset, some sort of "digital gold" or, worst, a "get rich scheme", it means it's failing.