Late-night is all about habit. Once you break an established pattern, you break habits. Some Jay fans—maybe not most, but maybe enough to make a difference—gravitated over to Dave last summer, and Jay cannot count on them back. Nothing personal: just habits.
Despite which, Jay could very likely beat Dave when he comes back anyway. But it may be by a smaller margin. And as opposed to in the ’90s, when Jay as a new host was able to grow his audience, I’m not sure that Jay, a known commodity at 11:35 for over two decades, has any room to grow. He might. But he might instead be starting the Tonight Show with as many viewers as he will ever have again.
And let’s be blunt: those viewers will be older. And if the past is a guide, they will continue to grow older. As Jay has pointed out, he always did better with young viewers than he got credit for, because his overall numbers were so high. But Conan made Tonight younger overnight, and it will age back—a big consideration for ad revenue—overnight. And again because of the force of habit, that audience will age.