Speculation, but well-founded speculation. Obsolete or not given ETOPS advances since the 1980s, the Secret Service has a bias toward four-engine solutions for long-haul presidential work. The 748 is the only remaining American-made plane that suits. The current 742s came in with Bush 42 in early 1989 and are now 25 years old. Expect replacements by the end of the decade which will keep the 747 in the public eye through 2050 or so. It is indeed widely believed that this is Boeing's main incentive to keep the 748 line going.