I would say, it will not become completely illegal and I also would not compare bitcoin to chemical or biological substances in the sense of criminalization.

Substances have direct effect on a person's biology, whether it is good or bad effect it does not matter, but it can make the argument emerge and thus justifications.

Bitcoin is a virtual object. It never has direct effect on people's biology. Also, it is unarguably evident that bitcoin is a more efficient means of transport for value. The only part that can build up an illegal aspect is that it can be used anonymously, thus also be used for money laundering and moving criminal money in large amounts.  Therefore the use of bitcoin might become strongly regulated, while it actually already is. All bitcoin traders need to follow the regulations set for a financial institution.