[RetroSpark]

The Sega Genesis did something similar - its hardware is a strict superset of the Master System's, so it can run SMS games natively.

However, unlike the Game Boy and GBA, the cartridge ports on the two systems are physically different (50 pins vs 64) so a cartridge adapter is still needed.

In that situation, I think an approach like the Super Game Boy's is probably better: include the old console's hardware in the adapter rather than in the new console, so that the new console's design need not be constrained by backward-compatibility.