Refuelling sats won't ever be a thing. There is no real need. Any refuelling vehicle will need it's own propulsion/rcs system and a docking scheme. Once docked and/or latched onto the target ... job done. The refuelling vessel becomes the new orbital manoeuvring system for the depleted sat. Cost wise, that makes more sense than having a large tanker bouncing around between clients. With the switch to ion propulsion total fuel may also not be the limiting factor. Ion thrusters can become very inefficient and/or useless after years in space. Flying up and bolting on a new rig is simpler than trying to actually service/replace such parts.