...to no longer be a threat to an unvaccinated person.
This isn't like the childhood vaccines, where we need herd immunity to protect the children who are too young to be vaccinated.
Provided that everyone who wants the vaccine can get it, I don't see the problem with letting everyone else make their own decision.
The current priority list of people is nowhere near 70% of the population anyway. Most people under 50 won't have the opportunity to be vaccinated for months. So this absolutely is a theoretical problem, at least for now.
Given that Covid is such a mild disease for most young people, I predict that by the time the elderly and vulnerable have been vaccinated, this will be a non-issue anyway.