do springs get soft over time

[stssilverado]

my 68 convertible sags in the ass. it's a monoleaf setup and the rear is 1" lower than the front. do springs get old and "soft"?

i'm looking at the hotchkis mono to multi leaf springs. has anyone had experience with the conversion? is there a better option (other than an expensive 4link)?

thanks for the help.