In the classic ideal of a romanticist, it would follow that Marianne should be the one to forgo the needs of a wealthy income, able to 'live on love' as it were. Austen has written it backwards was my first thought (although I did not think it an error) but I then came to understand that Marianne is living and thinking in the now, and now Willoughby - with a wealthy income- is the be all and end all of her romantic imaginings. So love - in her mind - must be with Willoughby alone. Of course I think Marianne's selfishness may also make her shallow (an idea which would shock her about herself) and her romantic ideals may REQUIRE a wealthy man. Perhaps if some footman had come along with everything Willoughby possessed in the way of feeling and education, except for his income, he may have been quite overlooked.