I guess they measure success of bitcoin to whether it will replace gold or fiat eventually. But even without doing those things, bitcoin is already successful.
People also ask that question too, if BTC will replace fiat and become the world's reserve currency, they forget that the world is governed by centralized governments, how would they adopt BTC that they cannot print or control for such a purpose. BTC transactions are also not instant and with its limited supply, it is not sufficient to fulfill the needs of the billions of people around the world.