If Airbus/Boeing took the longgggview they'd be working on an A380/B777 tanker for 2030......................
One A380 would be able to refuel every aircraft that the RAF will have by then in one go
May I point out that it's smaller size was a big reason why the KC-10 was chosen over the KC-747 back in the day. The amount of fuel available for offload is very often less important than the number of available offload points (tanker orbits). The bigger the tanker, the fewer tankers will be bought and so the fewer tanker orbits will be available. And if the tankers are forward deployed, as they often are by USAF, those tankers need to share precious ramp space with fighters, bombers, transports, and other tankers. USAF calls that MOG (maximum on ground). MOG is often a serious constraint in a forward theater. Because of their size, A380s are significantly constrained in the number of airfields they can operate in and out of even in airline operations, which would almost certainly make them a poor choice for a tanker, as would a 747-8.
The A350, 787, or maybe 777X would almost certainly make a better choice to replace KC-10s in the future. Although the 777X would likely be outsized for the job.
If "bigger" were indeed "better", then the MRTT would be based on the A330-300 rather than the -200 and there never would have been an A310 MRTT.