People who are comfortable with paper money will only use paper money even if bitcoin hits the mainstream as they will find easy way to spend their money, and paper money will take care of it.
These are some people who do not want to accept the change. They cant digest change from paper currency to Digital. If it is possible we have to educate them so that they get to know the benefits of this Bitcoin currency and hence they start using it.
Yes we all know that people resist change and its really difficult for them to accpet or switch to something new, as they will only do that only if they are hundred percent sure about it or else they will continue with fiat.