I dont think the fact that some really small dictatorships (are we one yet) have quite exotic presidential transports is not a reason for us to go the same route. Current arrangements fine and a sensible solution, France of course has a president who is much more akin to the US president than the UK PM and in some ways has even more authority than POTUS so he is likely to have something more exotique although the airframes are sometimes pretty old years wise. The queen is too old for long haultrips any more , Charles and Wills are unlikely to complain over the current arrangement. Must be a bit sad for Queenie though to lose the Royal yacht and the shiny VC10s she had for much of her reign but as we have all found out lately she doesnt actually do anything maybe she was a bit spoiled.
All politicians need their feet kept on the ground and it would do the US no harm at all if POTUS used an old KC135 or a Galaxy if size matters-after all the only real issues of importance on the presidential aircraft are secure as possible comms and a long endurance.
,In fact sometime like a C17 with good short field performance would be a much better choice than AF1