For every action, there is an equal and opposite reaction.
Make them all strong. Give all those potentials out there the power that was only ever meant for one, let the hundreds and thousands of girls in the world that can rise up do so, and fight, and become a slayer. Create an army of super-powerful young women, and travel everywhere, informing them of their power, sharing with them the knowledge that was once confined to a small select few individuals, and believe that maybe one day good will triumph over evil.
To expect no consequences is foolish. The vampires and demons grow stronger, multiplying exponentially. They react to the influx of new strong girls in the same way a persistent disease responds to antibiotics – by mutating, by adapting, by becoming even stronger. Evil is not inflexible. Evil can adapt. Evil has adapted.
They are the only ones left, of the old crowd. The reformed slayer, the one who was always in the shadow of her fairer counterpart. The sister, the one who was always the protected and never the protector. Things have changed now. They have emerged from the shadows. Buffy is dead, after all, truly dead this time, with no loopholes, no hidden clauses in an agreement, no possible way to return.
All they have is each other, something familiar to cling to. History is rewritten. Buffy becomes a saint in their retellings to each other late at night (so late it is closer to day, so late that even the vampires have disappeared from the streets). It is so much easier to think fondly of those who are gone.
Together, they are a force to be reckoned with. They are both aware that they could do many things now, things that even Willow, even Willow who almost destroyed the world, could not have accomplished. But they have learned. They will not tamper with the order of things. They have learned.