In ,
mcp6453 spewed forth:
My elderly in-laws have a couch that allows you to sink deeply into it
when you sit. Because of their physical infirmities, standing is
difficult. The frame of the couch is in tact. None of the frame is
broken or drooping. The cushions look fine, but I'm sure they have
lost some of their support. My theory is that the springs supporting
the seat cushions are worn.

Assuming that my theory is correct, does it make sense to try to get
the springs replaced? Is there a snowball's chance that a furniture
repair person or an upholsterer would be able to replace the springs?
The sofa is not a sleeper version. It's just a sofa.

The couch is in great condition otherwise, and the in-laws are not
amenable to buying new furniture. Any thoughts?

The springs probably need to be retied, which cannot be done properly
without recovering the sofa.
That is if it is coil springs and not no-sags(zig zag), I doubt seriously
that the springs need to be replaced.
I've never seen a coil spring wear out, unless it was improperly installed
and actually broke.
As others have suggested your first bet is to try a piece of plywood over
the deck under the cushions and raise the base.
Secondly, I would try a firmer core in the cushions.
If it's a quality piece, it would be worth it to fix it.