In ch 5 Anne thinks that in the event of Mrs Clay marring Sir Walter, Elizabeth would be "so much more to be pitied than herself" and Elizabeth herself acknowledges the fact that "I have a great deal more at stake... than anyone else..."

As you say at this point there doesn't seem to be any hope of Mr Elliot making Elizabeth Lady Elliot, however the danger would be Elizabeth's loss of status, which after 13 years of being mistress of Kellynch Hall, doing the honours and laying down the domestic law, leading the way to the chaise and four and being second only to Lady Russell in precedence (Ch 1) would certainly come hard to someone as proud as Elizabeth.

While there is no Lady Elliot, Elizabeth can enter further into the "years of danger" (Ch 1) with the comfort of having more importance than she would have as just plain Sir Walter's eldest spinster daughter.