Here are some facts:
The population of the US is rising faster than at any time in history.
The number of dentists is projected to fall in the next ten years, both in absolute numbers and especially against the population in general. By 2020 the dentist/population ratio will be the lowest since World War I.
There are still many multiple county areas in the US with no dentists at all serving them.
With state budgets what they are, it is getting harder to open a new dental school and attract faculty necessary to open one. As demand outpaces supply, salaries rise and academia becomes less attractive. This has the vicious cycle of further lowering entrants to dentistry.
The population is aging rapidly and much of the population over 50 in America had no access to flouride or other preventions for many years. They are more likely to require care but less likely to be edentulous. This bulge should last at least 30 more years, at current life expectancies.
The diet and consumption habits of younger Americans are abysmal. Soda in schools everywhere and the like are rotting a lot of teeth. This has undone a lot of the good that preventions were doing.
A GP can learn virtually anything. My GP does some ortho, endo, and prostho as well as general work. He is obviously very successful because he can keep a ton of profitable work for himself. New technologies make it easier for GPs to keep work "in-house".
Yes, a vaccine may make a nice sized long-term dent in the profession, but it would most widely reach those who don't need it. The kids who got the monthly appts for the vaccine would be the same ones who dutifully came in for perfect 15 minute checkups every six months. The kids who were most likely to be at risk would still not get it.
The biggest risk the profession faces is a takeover by managed care, leading to massive cuts in pay and quality of care. This could also allow the scourge of wild government sponsored immigration to take hold, as DMOs complain about being held hostage by dentists. The DMOs would seek tens of thousands of foreign dentists to come in and depress wages to the point of bankrupting the existing practices through a huge oversupply. Government lobbying is a powerful thing, just look at IT and the fake shortages they made up to depress wages.