That's a big assumption. There are millions of bitcoins to P2PK outputs, many of which get spent frequently throughout the years. Definitely people lost a fortune back in 2009 due to some accident, but it's no way millions. Perhaps few hundreds of thousands have been non-provably lost. Impossible to know exactly nor to approach it effectively.

Note that there are addresses with revealed public keys that do have a balance and aren't P2PK outputs, such as 1P5ZEDWTKTFGxQjZphgWPQUpe554WKDfHQ. Those are in the same danger as well if their owners don't move them to a quantum-safe address.