“Great Tzadikim have more zchusim to pass on.”
Again, you are privy as to how zechusim are tabulated, collected and allocated? You are certainly correct that great tzadikim do a lot more than the avg poshiteryid, however, you do not know how the ribbono shel olam “scores” their torah and maasim tovim, and how they are applied to later doros.
Personally, I am familiar with the maamer chazal to seek to marry a bas talmid chacham, the maamer chazal that a father should seek to marry his daughter to a talmid chacham, and even the maamar chazal about looking into the brothers of a potential wife. The only yichus chazal have told us to look out for are those that are psulai kahal. If you are familiar with a maamar chazal that one should seek a marriage partner who shtams from yichus, please share it. If all things were equal, then why not look at the elte zeida, otherwise the elte zeida could have been the gadol hador of a previous generation, but if his grandaughter is a shrew, all his zechusim wont help you (they certainly didnt help her, or her parents who raised her).