the issue between Lady Russell and Wentworth isn't so much whether he's realistic in his assessment of his chances, whether she's right to think he's a bad risk, etc. It's more fundamental than that: he's operating in a world in which you make your own way, you rise or fall essentially on your own merits (although of course it always helps to have connections). In a world like that, what's important is individual ability, drive, spirit. Lady Russell is accustomed to a world in which you can't expect to rise or fall that much: your position is fixed based on factors essentially outside of your own control, like birth, family, rank, inherited status. In that world, individual qualities may be the icing on the cake, but the cake itself was baked at birth. So it's not surprising that these two people can't really see each other's POV: they start from fundamentally different premises.