> But why would the existence of cars with better systems make driving vehicles like most people own today more expensive than currently?

Safer options will, as they become available, be disproportionately chosen by the safety conscious. This can, potentially, make the less-safe legacy option more dangerous (per use) than before the safer option became available, because the safest users of the less-safe option before the new one are users of the new option afterward, and no longer bringing the average risk of the old option down.

Conversely, if a significant proportion of the cars on the road are autonomous and safer than human-driven cars, then the accident rate for everyone could decline, making insurance cheaper for both autonomous and human-driven cars.

Wouldn't that make insurance cheaper for autonomous cars and more expensive for human-driven cars since humans would be more of a liability behind the wheel?

The comparison isn't between cars with assistive/autonomous systems and those without. It's between cars without those systems some decades hence and those without such systems today. Why should the existence of such systems make driving by humans more expensive than it is today?

Not necessarily. You could equally argue that self-driving cars will be more popular among bad drivers.

letting humans drive cars is insane. i'm not paying into your insurance pool.