Monero is surely a safe bet for the long term since it has a solid foundation and useful use cases. Speaking about the mainstream, I doubt that the majority of people care about privacy that much. People love convenience instead of privacy, and the way people achieve their privacy is time-consuming for the mainstream. Nevertheless, there will be a market for people who truly thrive for their own anonymity and privacy, and monero suits them, so in the long term, Monero would stand a chance.