Remember that Elizabeth regards Darcy as an ill-tempered man. From chapter 16: "I should take him, even on my slight acquaintance, to be an ill-tempered man."

And Darcy himself says:
My temper I dare not vouch for. (chpt 11).

Nothing in Darcy's letter has corrected that impression. Elizabeth certainly would have reason to be concerned for her future if she married a bad-tempered man. I never thought I would say this, but Elizabeth's views perhaps have in attitude something similar to Mr Collins' views on importance of temper to marriage:
"Pardon me for interrupting you, madam," cried Mr. Collins; "but if she is really headstrong and foolish, I know not whether she would altogether be a very desirable wife to a man in my situation, who naturally looks for happiness in the marriage state. If, therefore, she actually persists in rejecting my suit, perhaps it were better not to force her into accepting me, because if liable to such defects of temper, she could not contribute much to my felicity." (chpt 20)

The decision to marry someone or not is of course inherently a decision about the future. Elizabeth appears to worry about her future happiness, Mrs Bennet only about her future wealth - and of course, Mrs Bennet would not have to live with whoever Elizabeth married, so it's one thing for *her* to only worry about money, another for Elizabeth to do so.