It's currently estimated about 3 million people own some amount of Bitcoin.
That's about 0.3% of the population of Europe and North America.
If Bitcoin becomes mainstream, and say 50% of people in Europe and NA own Bitcoin, how much would it be worth?
The only certain thing is that it would be worth much more when it becomes mainstream. It can also be easily spent everywhere, even on groceries. It would also be taxed by that time and tax evasion using bitcoin would be illegal when that time comes. Probably when it becomes mainstream, being a bitcoin miner would be recognized as an official profession. There might even be courses that can be taken by college students related to bitcoin mining itself.