The two inputs nobody states when they quote a multiple

Neither is impossible on its own. Both simultaneously is the part worth flagging, because the usual defence of one implicitly assumes the other is conservative. This is not a timing argument. Multiples can hold implausible inputs for years. It is an argument about what the forward return distribution looks like from here, which is narrower and lower than the trailing experience. Invert the multiple and it resolves to a required return given a growth assumption. At current levels the implied combination is either a terminal growth rate above the long-run nominal economy or an equity risk premium near historic lows.