Bitcoin is a useful easy means to prepare for the future and old age. Start saving now, bitcoin can still increase 12 times before 15 years to this now.
Do not save the fiat because the government will scam you, they will devalue the fiat for their own benefit. Fiat look stable but fall in value after long time, bitcoin is volatile but increase in value over a long time.
Be wise and do not fall for the government scam fiat trap.
12 times in 15 years is quite an achievable target. Bitcoin has already generated unrealistic returns over the last decade.
12x is very realistic return and very much possible with bitcoin. Even I believe that most of our holdings should be in bitcoin.
Fiat is just for regular payments and day to day transactions and so it is better to keep only that much fiat money which is sufficient for daily use and invest the rest.
Investments can be diversified though. Not everything should be put into bitcoin but it can surely be a bigger part of our portfolio.