I always followed the classic advice of playing mainlines since they are supposedly best.
however while looking for a response to a common sideline I noticed something astonishing! The following moves score way better for white than any mainline. In all databases.
But 4 Qxd4 isn't a common side line. According to the Chess.com Master Games database, 99% of people play Nxd4.

Compared with 125,000 playing Nxd4, you see only 227 played Qxd4. It makes sense because it loses time to Nc6.
Of those 227 in the chess.com Master Game database, black wins 45% of the games and white won only 38%.
It doesn't lose immediately but I see no reason to give 6 Nc3 a !!.
Please let us know the names of those databases where you researched it.
I always followed the classic advice of playing mainlines since they are supposedly best.
however while looking for a response to a common sideline I noticed something astonishing! The following moves score way better for white than any mainline. In all databases.