Is there any reason in principle that large aircraft couldn't be converted to be controlled remotely?
Yes for reasons of hacking prevention. There is no way to be absolutely sure any interface can't be hacked to crash the plane remotely into a suburb.
Yes autopilots can follow a track of navigation marks to an airfield and can auto-land using the ILS.


But again there is no way to ensure only signals from people who have the best intentions with the plane can activate that mode.