There's lots of autopilot systems that smash into shit, sailboats are one of them, you can put your sailboat on autopilot and it's going to smash into stuff.
Samething with an airliner, autopilot will not avoid another aircraft.
They exist. But collision avoidance is far from a necessary retirement. E.g. my father in law's private single engine. Autopilot is basically cruise control, but you know...for a pilot
TCAS in a plane is like Automatic Emergency Braking (AEB) in a car. It's active 100% of the time in cars, not just when you're using super-cruise-control or other automation.
Of course TCAS is always enabled while in flight and is technically unrelated to autopilot. That doesn’t negate the point that many Airbus planes will automatically follow RAs when on autopilot if that is enabled by the flight crew.
