When Linda says "Life is a casting off" what does she mean?What type of rhetorical device is this in Death of a Salesman?

In Death of a Salesman the phrase
"Casting off" is used during a conversation where Linda and Willy are discussing their
sons, who have come back to live with them.

WILLY: Figure
it out. Work a lifetime to pay off a house. You
finally own it, and there’s
nobody to live in it.
LINDA: Well, dear, life is a casting off.
It’s always that way.
WILLY: No, no, some people- some people
accomplish something.

Casting off is a knitting term also
known as binding off.  It is used to define an edge such as the bottom of a sweater or
the top of a sock.  The stitch is used to secure the last row that you have completed so
you can concentrate on the next.    Linda, being a homemaker, is more likely to be using
this metaphor.  

In Linda's mind, each trouble, joy,
frustration, obstacle in life is like a row in knitting.  Eventually the row ends and
you cast it off and begin focusing on the next.  Linda fixes the holes in her stockings,
solving the problem- casting off.  She finds the hose under the water heater and takes
it- casting off.  He yells at her, she supports him when her sons come to her rescue-
casting off.  She suspects he is having an affair, but he always comes home to her-
casting off.  She steadfastly made the payments to the house despite their financial
hardships- casting off.  Her husband loses his job- casting off.  Finally, Willy commits
suicide, her boys promise to continue what he started, and Linda has a final moment with
her husband at his grave:  

LINDA: Forgive me, dear. I
can’t cry. I don’t know what it is, I
can’t cry. I don’t understand it. Why
did you ever do that? Help
me Willy, I can’t cry. It seems to me that you’re
just on another
trip. I keep expecting you. Willy, dear, I can’t cry. Why did
you
do it? I search and search and I search, and I can’t
understand
it, Willy. I made the last payment on the house today.
Today,
dear. And there’ll be nobody home. (A sob rises in her
throat.)
We’re free and clear. (Sobbing more fully, released.) We’re
free.
(Biff comes slowly toward her.) We’re free... We’re
free...

Casting off...