sirius black was meant to inherit the fortunes of both his grandparents, one through alphard and one through orion.
this is what could turn him into potentially the most powerful wizard in the lense of society. we know the malfoys were the richest when we were introduced to them, but that was in a world where the black heir was in prison.
this is what sells to me the idea that neither walburga or orion wanted to play the family game and leave the task up to cygnuss, since alphard didn’t played along either.
walburga and orion marriage was a powerful move. together they ensured the next heir to be powerful whilst securing the family line. but also, there’s a chance they had regulus not as the spare, or at least not in the sense for him to become the heir in case something happens with sirius, but more as to spare him of any duties.
they could’ve put all the expectations and pressure onto sirius and simultaneously free up his youngest to be who ever he wanted to be, an option they didn’t have.