Perforated Septum and Sunken Nose 20+ Years Post-Rhinoplasty. Causes?

It sounds like you have a saddle nose deformity. This can result from a septal perforation like you describe. Prior surgery can lead to a septal perforation, but other causes such at autoimmune disease, Wegener's disease, etc. should also be looked into as a possible cause of progressive worsening.

Depending on the perforation size your septum may be able to be repaired. The saddle nose deformity may also be reconstructed as well. Check out my link below to see an example of this.

There are lots of potential causes of a septal perforation including:-trauma-previous nasal surgery-infections-nose picking or manipulations-granulmatosis disease-nasal sprays

It appears that you have a saddle nose deformity - such as illustrated in the link below.  This is the result of loss of cartilage in the septum which can be caused by injury, medications, autoimmune diseases, or infection

BGT,

One of the world's experts in repair of septal perforations is located in Houston, TX - Dr. Russell Kridel.  He is a master in treatment of septal perforation and rhinoplasty in general.  You should have him take a look.
 

Certainly having prior surgery is a probable cause for septal perforation and resulting saddle nose deformity. However other disease conditions should be excluded. Wegener's Granulomatosis is a disease that can cause septal perforation. Sometimes medicated nasal sprays could lead to this although that is very rare. Long term prednisone use may have had some role as well. You also want to have an examination in the nose to rule out any other potential causes.