A real possibility already. Outside the U.K., with the long-established 12 ft-lbs limit.
Setting aside the historical precedents (Girandoni etc) in 1979, a top power air rifle made 11-15.
1980, the HW80 was gusting 20.
1990s, the Patriot, Eliminator and modern PCPs were hitting 30 ft-lbs in .22” and .25”.
Now, there is stuff out there in .357” and .45” making as much as 500 ft-lbs.
At some point, in unrestricted jurisdictions:
A) a criminal will kill someone with an air rifle that makes more power than a 9mm or 45ACP;
B) a mass-shooting nutter will do the same, just more than once;
C) pressure groups/politicians/media will discover that “BB guns” now make more power than a 45ACP. Especially if the next step in development involves magazine repeaters, real or fake semi-auto, etc.