100% reliable two way data comms is the key. Once that holy grail has been attained, its all over for the 2 pilot cockpit. They can build autopilots with all kinds of sensors to guide the aircraft to land without a Cat 3 ILS. An early but crude adopter is
https://discover.garmin.com/en-US/autonomi/
Think Tesla autopilots and how they manage in a complex, dynamic and constantly changing environment. Getting an aircraft from the gate at A to the gate at B is computational child's play in comparison.
When 2 way data comms reach that 100% reliability stage pilots will be replaced by former call centre operators in Mumbai who will each be managing dozens of aircraft simultaneously. Scary, hey?
CPDLC is the Trojan Horse. Indeed, its all very cool not having to use the HF but it's now technically possible for ATC to issue commands directly to the autopilot and only a question of when they will be allowed that capability.
The major obstacle will be marketing this new pilot-less, airline manager's wet dream to the general public. Price is their only consideration. Low enough and people will fly in anything - just look at the LCC's stooging around the skies of our northern neighbours - all packed to the gunwales with price conscious punters.