It would be slow, no doubt, but the variable cost to transport containers would be incredibly low. No energy used, no crew to pay. All you need after that is to have a small fleet of them, and a dock that can do autonomous loading and unloading.
For instance: - the sailcargo project (https://www.sailcargo.inc/) main ship has as many guest travellers than sailors (assuming "guests" means paying customers) - the ecoclipper500 project (https://ecoclipper.org/prototype/) also has room for "guests" - many of the traditional large sailboats are packed with paying guests and unpaid trainees as well
https://www.forbes.com/sites/nishandegnarain/2020/12/01/a-ne...
Large ocean going ships will never be fully autonomous. Humans are needed to conduct maintenance and repair, which is generally too complex to automate. But automation will reduce the crew needed for standing watch in the engine room and bridge.
Also maybe worth remembering that once close to the departing or destination harbour, any large cargo ship met with the harbour pilot team who goes onboard and take responsibility for manoeuvering the ship, so no ship would be unmanned the full course of its journey anyway ; only the easier, longer part.